Electrode, power storage device, and electronic device

ABSTRACT

A power storage device with high capacity or high energy density is provided. A highly reliable power storage device is provided. A long-life power storage device is provided. An electrode includes an active material, a first binder, and a second binder. The specific surface area of the active material is S [m 2 /g]. The weight of the active material, the weight of the first binder, and the weight of the second binder are a, b, and c, respectively. The solution of {(b+c)/(a+b+c)}×100÷S is 0.3 or more. The electrode includes a first film in contact with the active material. The first film preferably includes a region in contact with the active material. The first film preferably includes a region with a thickness of 2 nm or more and 20 nm or less. The first film contains a water-soluble polymer.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. The present invention relates to a process, a machine,manufacture, or a composition of matter. In particular, one embodimentof the present invention relates to a semiconductor device, a displaydevice, a light-emitting device, a power storage device, a storagedevice, a driving method thereof, or a manufacturing method thereof. Inparticular, one embodiment of the present invention relates to a powerstorage device and a manufacturing method thereof.

Note that a power storage device in this specification refers to everyelement and/or device having a function of storing electric power.

2. Description of the Related Art

In recent years, a variety of power storage devices, for example,secondary batteries such as lithium-ion secondary batteries, lithium-ioncapacitors, and air cells have been actively developed. In particular,demand for lithium-ion secondary batteries with high output and highenergy density has rapidly grown with the development of thesemiconductor industry, for electronic devices, for example, portableinformation terminals such as mobile phones, smartphones, and laptopcomputers, portable music players, and digital cameras; medicalequipment; next-generation dean energy vehicles such as hybrid electricvehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electricvehicles (PHEVs); and the like. The lithium-ion secondary batteries areessential as rechargeable energy supply sources for today's informationsociety.

An example of a lithium-ion battery includes at least a positiveelectrode, a negative electrode, and an electrolytic solution (PatentDocument 1).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2012-009418

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide apower storage device with high capacity. Another object of oneembodiment of the present invention is to provide a power storage devicewith high energy density. Another object of one embodiment of thepresent invention is to provide a highly reliable power storage device.Another object of one embodiment of the present invention is to providea long-life power storage device.

Another object of one embodiment of the present invention is to providea power storage device with reduced irreversible capacity. Anotherobject of one embodiment of the present invention is to provide a powerstorage device in which the decomposition reaction of an electrolyticsolution is inhibited and a decrease in capacity with increasing numberof charge and discharge cycles is prevented. Another object of oneembodiment of the present invention is to reduce or inhibit thedecomposition reaction of an electrolytic solution, which speeds up athigh temperature and prevent a decrease in charge and discharge capacityby charge and discharge at high temperature, in order to extend theoperating temperature range of a power storage device.

Another object of one embodiment of the present invention is to increasean yield of a power storage device. Another object of one embodiment ofthe present invention is to provide a novel power storage device, anovel electrode, or the like.

Note that the description of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is sn electrode including acurrent collector and an active material layer. The active materiallayer includes an active material and a first film. The first filmincludes a region in contact with the active material. The first filmincludes a region with a thickness of 2 nm or more and 20 nm or less.The first film contains a water-soluble polymer.

In the above structure, the active material is in the form of particles,and the specific surface area of the active material is preferablygreater than or equal to 0.2 m²/g and less than or equal to 7.0 m²/g. Inthe above structure, the water-soluble polymer preferably containscarboxymethyl cellulose, methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, or diacetyl cellulose. In the above structure,the first film preferably contains a stylene monomer or a butadienemonomer. In the above structure, the active material preferably containsgraphite.

Another embodiment of the present invention is an electrode including acurrent collector, an active material, a first binder, and a secondbinder. A variable A defined by Mathematical Formula 1 where S is thespecific surface area of the active material [m²/g], and a, b, and c arethe weight of the active material, the weight of the first binder, andthe weight of the second binder, respectively, is 0.3 or more. In theabove structure, the electrode preferably includes a first film incontact with the active material. The first film preferably includes aregion in contact with the active material. The first film preferablyincludes a region with a thickness of 2 nm or more and 20 nm or less.The first film preferably contains a water-soluble polymer.

$\begin{matrix}{A = {\frac{b + c}{a + b + c} \times {100 \div S}}} & {\mspace{11mu}\left\lbrack {{Mathematical}\mspace{14mu}{Formula}\mspace{11mu} 1} \right\rbrack}\end{matrix}$

In the above structure, the active material is in the form of particles,and the specific surface area S of the active material is preferablygreater than or equal to 0.2 m²/g and less than or equal to 7.0 m²/g. Inthe above structure, the first binder preferably contains carboxymethylcellulose, methyl cellulose, ethyl cellulose, hydroxypropyl cellulose,or diacetyl cellulose. In the above structure, the second binderpreferably contains a stylene monomer or a butadiene monomer. In theabove structure, the active material preferably contains graphite.

Another embodiment of the present invention is a power storage deviceincluding the electrode described above and a second electrode. Theelectrode described above has a function of operating as one of apositive electrode and a negative electrode. The second electrode has afunction of operating as the other of the positive electrode and thenegative electrode.

Another embodiment of the present invention is a power storage deviceincluding a first electrode and an electrolytic solution. The firstelectrode includes a current collector and an active material layer. Theactive material layer includes an active material, a first film, and asecond film. The first film includes a region in contact with the activematerial. The second film includes a region in contact with the firstfilm. The first film contains a water-soluble polymer. The second filmcontains lithium, fluorine, oxygen, and carbon. The electrolyticsolution contains lithium, fluorine, oxygen, and carbon.

In the above structure, the active material is in the form of particles,and the specific surface area of the active material is preferablygreater than or equal to 0.2 m²/g and less than or equal to 7.0 m²/g. Inthe above structure, the water-soluble polymer preferably containscarboxymethyl cellulose, methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, or diacetyl cellulose. In the above structure,the first film preferably contains a stylene monomer or a butadienemonomer. In the above structure, the active material preferably containsgraphite.

In the above structure, the power storage device preferably includes asecond electrode, the first electrode preferably has a function ofoperating as a negative electrode, the second electrode preferably has afunction of operating as a positive electrode, and R defined byMathematical Formula 2 is preferably 20 or more and 90 or less.

                         [Mathematical  Formula  2]$R = {\frac{{Positive}\mspace{14mu}{electrode}\mspace{14mu}{capacity}}{{Negative}\mspace{14mu}{electrode}\mspace{14mu}{capacity}} \times {100\mspace{14mu}\lbrack\%\rbrack}}$

Another embodiment of the present invention is an electronic deviceincluding the power storage device described above and a display device.

According to one embodiment of the present invention, a power storagedevice with high capacity can be provided. According to anotherembodiment of the present invention, a power storage device with highenergy density can be provided. According to one embodiment of thepresent invention, a highly reliable power storage device can beprovided. According to one embodiment of the present invention, a powerstorage device with a long lifetime can be provided.

One embodiment of the present invention can provide a power storagedevice with reduced irreversible capacity. One embodiment of the presentinvention can provide a power storage device in which a decompositionreaction of an electrolytic solution is inhibited and a decrease incapacity with increasing number of charge and discharge cycles isprevented. One embodiment of the present invention makes it possible toreduce or inhibit the decomposition reaction of an electrolyticsolution, which speeds up at high temperature, and to prevent a decreasein charge and discharge capacity in charge and discharge at hightemperature, in order to extend the operating temperature range of apower storage device.

According to one embodiment of the present invention, an yield of apower storage device can be increased. According to one embodiment ofthe present invention, a novel power storage device, a novel electrode,or the like can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily have all the effects listed above. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are an overhead view and a cross-sectional view of anelectrode, respectively;

FIG. 2 illustrates a thin storage battery;

FIGS. 3A and 3B illustrate thin storage batteries;

FIGS. 4A to 4C are cross-sectional views of an electrode;

FIGS. 3A and 5B illustrate a thin storage battery;

FIGS. 6A and 6B illustrate a thin storage battery;

FIG. 7 illustrates a thin storage battery;

FIGS. 8A to 8C illustrate the radius of curvature of a surface;

FIGS. 9A to 9D illustrate the radius of curvature of a film;

FIGS. 10A and 10B illustrate a coin-type storage battery;

FIGS. 11A and 11B illustrate a cylindrical storage battery;

FIGS. 12A to 12C illustrate examples of power storage devices;

FIGS. 13A to 13C illustrate an example of a power storage device;

FIGS. 14A and 14B illustrate an example of a power storage device;

FIGS. 15A1, 15A2, 15B1, and 15B2 illustrate examples of power storagedevices;

FIGS. 16A and 16B illustrate examples of power storage devices;

FIGS. 17A to 17G illustrate examples of electronic devices;

FIGS. 18A to 18C illustrate an example of an electronic device;

FIG. 19 illustrate examples of electronic devices;

FIGS. 20A and 20B illustrate examples of electronic devices;

FIGS. 21A and 21B show ToF-SIMS analysis results of electrodes;

FIGS. 22A to 22C show cross-sectional TEM observation results of anelectrode;

FIGS. 23A to 23D show a STEM observation result and EDX analysisresults;

FIGS. 24A and 24B are graphs each showing charge and dischargeefficiency;

FIGS. 25A1, 25A2, 25B1, 25B2, 25C1, and 25C2 are graphs each showingcharge and discharge curves;

FIGS. 26A and 26B are graphs each showing the relation between chargeand discharge cycles and discharge capacity;

FIG. 27 is a graph showing the relation between charge and dischargecycles and discharge capacity;

FIGS. 28A to 28C are graphs each showing charge and dischargecharacteristics;

FIG. 29 is a graph showing charge and discharge characteristics;

FIGS. 30A to 30C show cross-sectional TEM observation results of anelectrode;

FIGS. 31A and 31B show cross-sectional TEM observation results of theelectrode;

FIGS. 32A to 32C show EELS analysis results of the electrode;

FIGS. 33A to 33C show EELS analysis results of the electrode;

FIGS. 34A and 34B show cross-sectional TEM observation results of anelectrode;

FIGS. 35A and 35B are graphs each showing discharge ratecharacteristics;

FIGS. 36A and 36B are graphs each showing discharge temperaturecharacteristics;

FIGS. 37A and 37B are graphs each showing the relation between chargeand discharge cycles and discharge capacity;

FIG. 38 is a graph showing the relation between capacity ratio andgradient;

FIG. 39 is a graph showing the relation between discharge capacity andthe number of times of bending of each power storage device;

FIG. 40 is a photograph of a bend tester;

FIG. 41 illustrates an operation example of a power storage device; and

FIG. 42 illustrates an operation example of a power storage device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and examples of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the description of theembodiments and examples and it is easily understood by those skilled inthe art that the mode and details can be changed variously. Accordingly,the present invention should not be interpreted as being limited to thedescription of the embodiments below.

Note that in drawings used in this specification, the thicknesses offilms, layers, and substrates and the sizes of components (e.g., thesizes of regions) are exaggerated for simplicity in some cases.Therefore, the sizes of the components are not limited to the sizes inthe drawings and relative sizes between the components.

Note that the ordinal numbers such as “first” and “second” in thisspecification and the like are used for convenience and do not denotethe order of steps, the stacking order of layers, or the like.Therefore, for example, description can be made even when “first” isreplaced with “second” or “third”, as appropriate. In addition, theordinal numbers in this specification and the like are not necessarilythe same as those which specify one embodiment of the present invention.

Note that in structures of the present invention described in thisspecification and the like, the same portions or portions having similarfunctions are denoted by common reference numerals in differentdrawings, and descriptions thereof are not repeated. Further, the samehatching pattern is applied to portions having similar functions, andthe portions are not especially denoted by reference numerals in somecases.

Note that in this specification and the like, a positive electrode and anegative electrode for a power storage device may be collectivelyreferred to as a power storage device electrode; in this case, the powerstorage device electrode refers to at least one of the positiveelectrode and the negative electrode for the power storage device.

Here, a charge rate and a discharge rate will be described. For example,in the case of charging a secondary battery with a certain capacity [Ah]at a constant current, a charge rate of 1 C means the current value I[A] with which charging is terminated in exactly 1 h, and a charge rateof 0.2 C means I/5 [A] (i.e., the current value with which charging isterminated in exactly 5 h). Similarly, a discharge rate of 1 C means thecurrent value I [A] with which discharging is ended in exactly 1 h, anda discharge rate of 0.2 C means I/5 [A] (i.e., the current value withwhich discharging is ended in exactly 5 h).

Embodiment 1

In this embodiment, an electrode of one embodiment of the presentinvention and a method for manufacturing the electrode will bedescribed.

[Structure of Electrode]

FIG. 1A is an overhead view of an electrode 100, and FIG. 1B is across-sectional view of a portion surrounded by a broken line in FIG.1A. The electrode 100 has a structure in which an active material layer102 is provided over a current collector 101. Although the activematerial layers 102 are provided such that the current collector 101 issandwiched therebetween, the active material layer 102 may be formedover only one surface of the current collector 101. The active materiallayer 102 includes an active material.

There is no particular limitation on the current collector 101 as longas it has high conductivity without causing a significant chemicalchange in a power storage device. For example, the current collector 101can be formed using a metal such as gold, platinum, zinc, iron, nickel,copper, aluminum, titanium, tantalum, or manganese, an alloy thereof(e.g., stainless steel), sintered carbon, or the like. Alternatively,copper or stainless steel that is coated with carbon, nickel, titanium,or the like can be used to form the current collector 101.Alternatively, the current collector 101 can be formed using an aluminumalloy to which an element that improves heat resistance, such assilicon, neodymium, scandium, or molybdenum, is added. Stillalternatively, a metal element that forms silicide by reacting withsilicon can be used. Examples of the metal element that forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, andthe like. The current collector 101 can have any of various shapesincluding a foil-like shape, a plate-like shape (sheet-like shape), anet-like shape, a cylindrical shape, a coil shape, a punching-metalshape, a porous shape, and a shape of non-woven fabric as appropriate.Alternatively, the current collector 101 can have an expanded-metalshape as a net-like shape, for example. The current collector 101 may beformed to have micro irregularities on the surface thereof in order toenhance adhesion to the active material layer, for example. The currentcollector 101 preferably has a thickness of 5 μm to 30 μm inclusive.

The positive electrode active material layer 102 includes the activematerial. An active material refers only to a material that relates toinsertion and extraction of ions that carriers. In this specificationand the like, a material that is actually an “active material” and thematerial including a conductive additive, a binder, and the like arecollectively referred to as an active material layer.

In the case where the active material is a negative electrode activematerial, for example, a carbon-based material, an alloy-based material,or the like can be used.

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, and carbon black.

Examples of graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, and pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal (higher than or equal to 0.1 V and lower than or equal to 0.3 Vvs. Li/Li⁺) when lithium ions are intercalated into the graphite (whilea lithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage. Inaddition, graphite is preferred because of its advantages such asrelatively high capacity per unit volume, small volume expansion, lowcost, and higher level of safety than that of a lithium metal.

For the negative electrode active material, a material which enablescharge-discharge reactions by an alloying reaction and a dealloyingreaction with lithium can be used. A material containing at least one ofGa, Si, Al, Si, Ge, Sn, Pb, Sb, Bi, Ag, Au, Zn, Cd, In, and the like canbe used, for example. Such elements have higher capacity than carbon. Inparticular, silicon has a significantly high theoretical capacity of4200 mAh/g. For this reason, silicon is preferably used as the negativeelectrode active material. Examples of an alloy-based material usingsuch elements include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂, Mg₂Sn, SnS₂, V₂Sn₃,FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃, LaSn₃,La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like. Here, the alloy-basedmaterial refers to a material that enables charge-discharge reactions byan alloying reaction and a dealloying reaction with lithium, a materialthat enables charge-discharge reactions by forming a bond with lithium,or the like.

Alternatively, for the negative electrode active materials, an oxidesuch as titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, for the negative electrode active materials,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive materials and thus the negative electrode active materials can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedfor the negative electrode active materials; for example, a transitionmetal oxide which does not form an alloy with lithium, such as cobaltoxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used.Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoSn_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃. Note that any of the fluorides can be used as a positive electrodeactive material because of its high potential.

The reaction potential of the negative electrode active material ispreferably as low as possible, in which case the voltage of the powerstorage device can be high. On the other hand, when the potential islow, power of reducing an electrolytic solution is increased, so that anorganic solvent or the like in an electrolytic solution might besubjected to reductive decomposition. The range of potentials in whichthe electrolysis of an electrolytic solution does not occur is referredto as a potential window. The electrode potential of the negativeelectrode needs to be within a potential window of an electrolyticsolution; however, the potentials of many active materials used fornegative electrodes of lithium-ion secondary batteries and lithium-ioncapacitors are out of the potential windows of almost all electrolyticsolutions. Specifically, materials with low reaction potentials such asgraphite and silicon can increase the voltage of storage batteries butare likely to cause the reductive decomposition of electrolyticsolutions.

Meanwhile, a positive electrode active material currently used for apositive electrode of a lithium-ion secondary battery might cause thedecomposition of an electrolytic solution at high temperature and athigh voltage.

In the case where the active material is a positive electrode activematerial, a material into and from which lithium ions can beintercalated and deintercalated can be used; for example, a materialhaving an olivine crystal structure, a layered rock-salt crystalstructure, a spinel crystal structure, or a NASICON crystal structure,or the like can be used.

As the positive electrode active material, a compound such as LiFeO₂,LiCoO₂, LiNiO₂, or LiMn₂O₄, V₂O₅, Cr₂O₅, or MnO₂ can be used.

Alternatively, lithium-containing complex phosphate with an olivinecrystal structure (LiMPO₄ (general formula) (M is one or more of Fe(II),Mn(II), Co(II), and Ni(II))) can be used. Typical examples are lithiummetal phosphate compounds such as LiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄,LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1),LiFe_(e)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

Alternatively, lithium-containing complex silicate such asLi_((2-j))MSiO₄ (general formula) (M is one or more of Fe(II), Mn(II),Co(II), and Ni(II); 0≤j≤2) may be used. Typical examples are lithiumsilicate compounds such as Li_((2-f))FeSiO₄, Li_((2-f))NiSiO₄,Li_((2-f))CoSiO₄, Li_((2-f))MnSiO₄, Li_((2-f))Fe_(k)Ni_(l)SiO₄,Li_((2-f))Fe_(k)Co_(l)SiO₄, Li_((2-f))Fe_(k)Mn_(l)SiO₄,Li_((2-f))Ni_(k)Co_(l)SiO₄, Li_((2-f))Ni_(k)Mn_(l)SiO₄ (k+l<1, 0<k<1,and 0<l<1), Li_((2-f))Fe_(m)Ni_(n)Co_(q)SiO₄,Li_((2-f))Fe_(m)Ni_(n)Mn_(q)SiO₄, Li_((2-f))Ni_(m)Co_(n)Mn_(q)SiO₄(m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2-f))Fe_(r)Ni_(s)Co_(f)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a NASICON compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, Nb, or Al, X=S, P,Mo, W, As, or Si) can be used for the active material. Examples of theNASICON compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃, and Li₃Fe₂(PO₄)₃. Furtheralternatively, a compound expressed by Li₂MPO₄F, Li₂MP₂O₇, or Li₅MO₄(general formula) (M=Fe or Mn), a perovskite fluoride such as NaF₃ andFeF₃, a metal chalcogenide (a sulfide, a selenide, or a telluride) suchas TiS₂ and MoS₂, a material with an inverse spinel structure such asLiMVO₄, a vanadium oxide (V₂O₅, V₆O₁₃, LiV₃O₈, or the like), a manganeseoxide, an organic sulfur compound, or the like can be used as thepositive electrode active material.

In the case where carrier ions are alkali metal ions other than lithiumions, or alkaline-earth metal ions, a compound containing carriers suchas an alkali metal (e.g., sodium and potassium) or an alkaline-earthmetal (e.g., calcium, strontium, barium, beryllium, and magnesium)instead of lithium of the lithium compound, the lithium-containingcomplex phosphate, or the lithium-containing complex silicate may beused as the positive electrode active material.

The average particle size of the positive electrode active material ispreferably, for example, greater than or equal to 5 nm and less than orequal to 50 μm.

For example, lithium-containing complex phosphate having an olivinecrystal structure used for the positive electrode active material has aone-dimensional lithium diffusion path, so that lithium diffusion isslow. The size of a primary particle of foe active material is thuspreferably reduced to increase the charge and discharge rate.Furthermore, the specific surface area of the active material ispreferably increased. The average size of primary particles ispreferably, for example, greater than or equal to nm and less than orequal to 1 μm. The specific surface area is preferably, for example,greater than or equal to 10 m²/g and less than or equal to 50 m²/g.

A positive electrode active material having an olivine crystal structureis much less likely to be changed in the crystal structure by charge anddischarge and has a more stable crystal structure than, for example, anactive material having a layered rock-salt crystal structure. Thus, apositive electrode active material having an olivine crystal structureis stable toward operation such as overcharge. The use of such apositive electrode active material allows fabrication of a highly safepower storage device.

The active material layer 102 may include a conductive additive.Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber, isotropicpitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbonnanotube can be formed by, for example, a vapor deposition method. Otherexamples of the conductive additive include carbon materials such ascarbon black (acetylene black (AB)) and graphene. Alternatively, metalpowder or metal fiber of copper, nickel, aluminum, silver, gold, or thelike, a conductive ceramic material, or the like can be used.

Flaky graphene has an excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength. For this reason, the use of graphene as theconductive additive can increase the points and the area where thenegative electrode active materials are in contact with each other.

Note that graphene in this specification includes single-layer grapheneand multilayer graphene including two to hundred layers. Single-layergraphene refers to a one-atom-thick sheet of carbon molecules having nbonds. Graphene oxide refers to a compound formed by oxidation of suchgraphene. When graphene oxide is reduced to form graphene, oxygencontained in the graphene oxide is not entirely released and part of theoxygen remains in graphene. When graphene contains oxygen, theproportion of oxygen, which is measured by X-ray photoelectronspectroscopy (XPS), is higher than or equal to 2 at % and lower than orequal to 20 at %, preferably higher than or equal to 3 at % and lowerthan or equal to 15 at %.

The active material layer 102 preferably includes a binder, morepreferably a binder that contains water-soluble polymers. The activematerial layer 102 may include a plurality of kinds of binders. A binderthat can be included in the active material layer 102 will be describedbelow.

The binder preferably contains water-soluble polymers. As thewater-soluble polymers, a polysaccharide or the like can be used. As thepolysaccharide, a cellulose derivative such as carboxymethyl cellulose(CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose,diacetyl cellulose, or regenerated cellulose, starch, or the like can beused.

Here, water-soluble polymers can be dissolved in water and thus canadjust and stabilize the viscosity of slurry for an electrode when theslurry is formed. Furthermore, water-soluble polymers facilitatedispersion of other materials, here, the active material and othermaterials such as a binder and a conductive additive, in the slurry. Theslurry is finally applied and then dried, so that an electrode isobtained. Note that “something can be dissolved in water” means that afunctional group of a polymer can be ionized in water, for example,here.

Here, water-soluble polymers do not necessarily dissolve only in water,and polymers that dissolve in a solvent other than water may be used.For example, polymers are dissolved in a polar solvent and an activematerial and other materials are dispersed in the mixture to formslurry. Alternatively, polymers that can be dissolved only in a solventother than water may be used.

As the binder, a rubber material such as styrene-butadiene rubber (SBR),styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber,butadiene rubber, or ethylene-propylene-diene copolymer is preferablyused. Any of these rubber materials is more preferably used incombination with the aforementioned water-soluble polymers. Since theserubber materials have rubber elasticity and easily expand and contract,it is possible to obtain a highly reliable electrode that is resistantto stress due to expansion and contraction of an active material bycharge and discharge, bending of the electrode, or the like. On theother hand, the rubber materials have a hydrophobic group and thus areunlikely to be soluble in water in some cases. In such a case, particlesare dispersed in an aqueous solution without being dissolved in waterand, so that increasing the viscosity of slurry up to the viscositysuitable for application to form the electrode might be difficult.Water-soluble polymers having an excellent function of adjustingviscosity, such as a polysaccharide, can moderately increase theviscosity of the solution and can be uniformly dispersed together with arubber material. Thus, a favorable electrode with high uniformity (e.g.,an electrode with uniform electrode thickness or electrode resistance)can be obtained.

As the binder, a material such as polystyrene, poly(methyl acrylate),poly(methyl methacrylate) (PMMA), sodium polyacrylate, polyvinyl alcohol(PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide,polyvinyl chloride, polytetrafluoroethylene, polyethylene,polypropylene, isobutylene, polyethylene terephthalate, nylon,polyvinylidene fluoride (PVDF), or polyacrylonitrile (PAN) can be used.

A single binder may be used or plural kinds of binders may be used incombination.

For example, a binder having high adhesion or high elasticity and abinder having a significant viscosity modifying effect may be used incombination. As the binder having a significant viscosity modifyingeffect, for example, a water-soluble polymer is preferably used. Anexample of a water-soluble polymer having an especially significantviscosity modifying effect is the above-mentioned polysaccharide; forexample, a cellulose derivative such as carboxymethyl cellulose (CMC),methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetylcellulose, or regenerated cellulose, or starch can be used.

Note that a cellulose derivative such as carboxymethyl cellulose obtainsa higher solubility when converted into a salt such as a sodium salt oran ammonium salt of carboxymethyl cellulose, and accordingly, easilyexerts an effect as a viscosity modifier. The high solubility can alsoincrease the dispersibility of an active material and other componentsin the formation of slurry for an electrode. In this specification,cellulose and a cellulose derivative used as a binder of an electrodeinclude salts thereof.

The water-soluble polymers stabilize viscosity by being dissolved inwater and allow stable dispersion of the active material and a secondbinder such as styrene-butadiene rubber in an aqueous solution.Furthermore, a water-soluble polymer is expected to be easily and stablyadsorbed to an active material surface because it has a functionalgroup. Many cellulose derivatives such as carboxymethyl cellulose havefunctional groups such as a hydroxyl group and a carboxyl group. Becauseof functional groups, polymers are expected to interact with each otherand cover an active material surface in a large area.

In the case where the binder covering or being in contact with theactive material surface forms a film, the film is expected to serve as apassivation film to suppress the decomposition of the electrolyticsolution. Here, the passivation film refers to a film without electricconductivity or a film with extremely low electric conductivity, and caninhibit the decomposition of an electrolytic solution at a potential atwhich a battery reaction occurs in the case where the passivation filmis formed on the active material surface, for example. It is preferredthat the passivation film can conduct lithium ions while suppressingelectric conduction.

In a specific example described here, a cellulose derivative having anexcellent viscosity modifying property is used as the binder. As thecellulose derivative, sodium carboxymethyl cellulose (hereinafterabbreviated to CMC-Na) is used. It is highly probable that CMC-Nacovering the active material surface can serve as the passivation film.The passivation film prevents decomposition or the like due to areaction of an electrolytic solution at the active material surface.Now, suppose that a material having rubber elasticity, such asstyrene-butadiene rubber (hereinafter abbreviated to SBR), is used asanother binder. Since a polymer containing a styrene monomer unit or abutadiene monomer unit, such as SBR, has rubber elasticity and easilyexpands and contracts, a highly reliable electrode that is resistant tostress due to expansion and contraction of an active material by chargeand discharge, bending of the electrode, or the like can be obtained. Onthe other hand, SBR has a hydrophobic group and thus might be slightlysoluble in water. Thus, particles are dispersed in an aqueous solutionwithout being dissolved in water, so that increasing the viscosity ofslurry up to the viscosity suitable for application to form theelectrode might be difficult. Meanwhile, when CMC-Na, which has anexcellent viscosity modifying property, is used, the viscosity of asolution can be increased moderately. By mixing CMC-Na with the activematerial and SBR in a solution, they are uniformly dispersed, so that afavorable electrode having high uniformity, specifically, an electrodehaving high uniformity in electrode thickness or electrode resistancecan be obtained. By being uniformly dispersed, SBR as well as CMC-Namight cover or be in contact with a surface of the active material. Inthis case, SBR may also contribute to a function as a passivation film.

When the active material is a negative electrode active material, thereaction potential is low and thus an electrolytic solution might bereductively decomposed as described above. The battery reactionpotentials of graphite, silicon, and the like are very close to theredox potential of lithium, so that graphite, silicon, and the likeparticularly significantly cause the decomposition of an electrolyticsolution in many cases. Thus, a binder covering the active materialsurface preferably forms a film or a film-like binder is preferably incontact with the active material surface to serve as a passivation filmso that the decomposition of an electrolytic solution is inhibited.Inhibiting the decomposition of an electrolytic solution leads tosuppression of generation of a gas caused by the decomposition. Thegeneration of a gas increases, for example, an unreacted active materialin a negative electrode; accordingly, the effectual current density isincreased and a voltage drop is increased. This increases thepossibility of lithium deposition. The use of the electrode of oneembodiment of the present invention as the negative electrode enablessolution of these problems.

Presumably, the smaller the specific surface area of the active materialis, the higher the proportion of the binder covering the surface or theproportion of the area of a region of the entire surface that is incontact with the film-like binder is. Thus, the proportion of the binderis optimized in accordance with the specific surface area of the activematerial, whereby a more reliable electrode can be fabricated.

[Fabricating Method of Electrode]

Next, a method for manufacturing the electrode 100 of one embodiment ofthe present invention will be described.

In order to form the active material layer 102, first, slurry is formed.The slurry can be formed in such a manner that the above-describedmaterial to which a conductive additive, a binder, and the like areadded as appropriate is mixed with a solvent, for example. As thesolvent, for example, water or N-methyl-2-pyrrolidone (NMP) can be used.Water is preferably used in terms of the safety and cost. With the useof a water-soluble polymer as the binder, slurry with an appropriateviscosity for application can be formed. In addition, slurry with highdispersibility can be formed. Accordingly, the binder can favorablycover with or be in contact with a surface of the active material. Informing the slurry, by kneading the active material and thewater-soluble polymer and then adding a solvent and other materials, theslurry with highly stable viscosity can be formed. It is also possibleto increase the dispersibility of the materials. Furthermore, thesurface of the active material can be easily coveted with the binder.

The case where the electrode 100 is a negative electrode of a storagebattery will be described as an example. Here, an example will bedescribed in which graphite is used as a negative electrode activematerial, CMC-Na and SBR are used as binders, and water is used as asolvent.

Graphite and CMC-Na are mixed in a mixer or the like to obtain amixture. In this case, graphite may be added to an aqueous solution ofCMC-Na after CMC-Na is dissolved in water to prepare the aqueoussolution of CMC-Na. The case where graphite is added to the aqueoussolution of CMC-Na is more preferred to the case where graphite is addedto water because cohesion of graphite and the like can be inhibited toenable uniform mixing.

Alternatively, graphite powder and CMC-Na powder may be mixed in a mixeror the like to be dispersed and then water may be added to the mixture.

Both the above cases are preferred because cohesion of graphite, whichis the active material, can be weaken and dispersibility of graphite andCMC-Na can be improved when a small amount of water is added and mixing(kneading) in a high viscosity state is performed to obtain a mixture inthe form of a paste.

After the kneading is performed, water may further be added and mixingmay be performed.

Then, SBR is added and mixing is performed using a mixer or the like.Here, an SBR dispersion liquid in which water has been mixed ispreferably added to the mixture, in which case cohesion of SBR can besuppressed unlike in the case where SBR powder is added. Furthermore,dispersibility of other materials and SBR is improved in some cases.

Next, the pressure in the mixer containing this mixture may be reducedto perform degasification. Through the above steps, favorable slurry inwhich graphite, CMC-Na, and SBR are uniformly dispersed can be formed.

Note that the order of mixing graphite, CMC-Na, and SBR is not limitedto the above. All the materials may be added at a time and mixed.

Here, any of a variety of mixers can be used as the mixer. For example,a planetary mixer, a homogenizer, or the like can be used.

The polymerization degree of CMC-Na that is used is preferably, forexample, higher than or equal to 100 and lower than or equal to 1000,more preferably higher than or equal to 500 and lower than or equal to900, still more preferably higher than or equal to 600 and lower than orequal to 800. In the case of forming a 1 wt % CMC-Na aqueous solution,the viscosity of the aqueous solution is preferably higher than or equalto 150 mPa·s and lower than or equal to 2000 mPa·s, more preferablyhigher than or equal to 200 mPa·s and lower than or equal to 1000 mPa·s,still more preferably higher than or equal to 300 mPa·s and lower thanor equal to 300 mPa·s. The Na content of GMC-Na after drying ispreferably, for example, higher than or equal to 5 wt % and lower thanor equal to 10 wt %, more preferably higher than or equal to 6.5 wt %and lower than or equal to 8.5%. Furthermore, the molecular weight ofCMC-Na that is used is preferably, for example, 130000 to 190000.

Here will be described examples of favorable proportions and a favorablespecific surface area of an active material when the electrode of oneembodiment of the present invention includes the active material, afirst binder, and the second binder and water-soluble polymers are usedas the first binder. The weight of the active material included in theelectrode is a, the weight of the first binder is b, the weight of thesecond binder is c, and the specific surface area of the active materialis S [m²/g], A variable B defined by Mathematical Formula (3) is, forexample, preferably 0.15 or more, more preferably 0.3 or more, stillmore preferably 0.5 or more.

$\begin{matrix}{B = {\frac{b}{a + b + c} \times {100 \div S}}} & {\mspace{11mu}\left\lbrack {{Mathematical}\mspace{14mu}{Formula}\mspace{14mu} 3} \right\rbrack}\end{matrix}$

When the weight of the active material included in the electrode is a,the weight of the first binder is b, the weight of the second binder isc, and the specific surface area of the active material is S [m²/g], thefavorable relation between the proportions and the specific surface areaof the active material is as follows: A defined by Mathematical Formula4 is preferably 0.3 or more, more preferably 1 or more, still morepreferably 2 or more. Particularly when if is 1 or more and theelectrode is used as a negative electrode of a storage battery, thestorage battery can have significantly excellent cycle characteristics.

$\begin{matrix}{A = {\frac{b + c}{a + b + c} \times {100 \div S}}} & {\mspace{11mu}\left\lbrack {{Mathematical}\mspace{14mu}{Formula}\mspace{14mu} 4} \right\rbrack}\end{matrix}$

Here, for example, graphite is particularly preferably used as theactive material, and polymers containing styrene monomers or butadienemonomers are particularly preferably used as the second binder.

The specific surface area of the active material that is used ispreferably greater than or equal to 0.2 m²/g and less than or equal to7.0 m²/g, more preferably greater than or equal to 0.5 m²/g and lessthan or equal to 3.0 m²/g, still more preferably greater than or equalto 0.5 m²/g and less than or equal to 2.5 m²/g, for example.

Here, in the case of using, for example, CMC-Na as water-solublepolymers, the proportion of CMC-Na with respect to the sum of theweights of the active material, CMC-Na, and the second binder ispreferably higher than or equal to 0.5 wt % and lower than 10 wt %, morepreferably higher than or equal to 1 wt % and lower than or equal to 7wt % in terms of easiness of electrode fabrication, stabilization ofelectrode characteristics, reduction in electrode resistance, or thelike.

For example, when the proportion of CMC-Na is less than 1 wt %,non-uniform application is likely to occur (the thickness uniformity ispoor, so that a thin portion is locally formed) in some cases. Thenon-uniform application is caused by an increase in viscosity due todrying of the slurry (volatilization of the solvent), for example. Ifthe proportion of CMC-Na is higher than 7 wt %, for example, thefluidity of the slurry might decrease. The proportion of CMC-Na wiferespect to fee sum of fee weights of fee active material, CMC-Na, andthe second binder is feus preferably higher than or equal to 0.5 wt %and lower than 10 wt %, more preferably higher than or equal to 1 wt %and lower than or equal to 7 wt %.

Here, in fee case where fee binder covering or being in contact with theactive material surface forms a film, fee film is expected to serve as apassivation film to inhibit fee decomposition of an electrolyticsolution. Such an effect may be obtained even when CMC-Na or SBR is notin a film form. A porous film of CMC-Na or SBR may be formed.

Any of the conductive additives listed above is preferably used, inwhich case a more highly conductive electrode can be fabricated. Stepsfor forming slurry for the electrode by mixing carbon fiber as aconductive additive, graphite, CMC-Na, and SBR will be described belowas an example. Examples of the carbon fiber include mesophasepitch-based carbon fiber, isotropic pitch-based carbon fiber, carbonnanofiber, and carbon nanotube. Carbon nanotube can be formed by, forexample, a vapor deposition method.

First, graphite and carbon fiber are mixed in a mixer or the like. Atthis time, it is preferred that graphite, carbon fiber, and a smallamount of water be kneaded (mixed in a high viscosity state), in whichcase graphite and carbon fiber may be easily dispersed uniformly.

Next, CMC-Na is added and mixing is performed using a mixer or the liketo obtain a mixture. At this time, a CMC-Na aqueous solution prepared bymixing CMC-Na with water in advance is preferably added, in which casecohesion of CMC-Na can be prevented. When water is added prior to theaddition of CMC-Na, the viscosity is lowered, damaging the dispersivestate of graphite and carbon fiber in some cases.

After that, water may further be added and mixing may be performed.

Then, SBR is added and mixing is performed using a mixer or the like.Here, an SBR dispersion liquid in which water has been mixed ispreferably added to the mixture, in which case cohesion of SBR can besuppressed unlike in the case where SBR powder is added. Furthermore,dispersibility of other materials and SBR is improved in some cases.

Next, the pressure in the mixer containing this mixture may be reducedto perform degasification. Through the above steps, favorable slurry inwhich graphite, carbon fiber, CMC-Na, and SBR are uniformly dispersedcan be formed.

Note that the order of mixing graphite, carbon fiber, CMC-Na, and SBR isnot limited to the above. All the materials may be added at a time andmixed.

As an example, another fabricating method will be described. First,graphite powder, carbon fiber powder, and CMC-Na powder are mixed in amixer or the like. Then, water is added in the state where the materialsare mixed, and mixing is further performed.

The above case is preferred because cohesion of graphite, which is theactive material, can be weaken and dispersibility of graphite, carbonfiber, and CMC-Na can be improved when a small amount of water is addedand mixing (kneading) in a high viscosity state is performed to obtain amixture in the form of a paste.

After the kneading is performed, water may further be added and mixingmay be performed.

Then, SBR is added and mixing is performed using a mixer or the like.Here, an SBR dispersion liquid in which water has been mixed ispreferably added to the mixture, in which case cohesion of SBR can besuppressed unlike in the case where SBR powder is added. Furthermore,dispersibility of other materials and SBR is improved in some cases.

After that, degasification may be performed. Through the above steps,favorable shiny in which graphite, carbon fiber, CMC-Na, and SBR areuniformly dispersed can be formed.

CMC-Na and SBR are uniformly dispersed, whereby when these binders coverthe active material surface and form a film, the film does not becometoo thick. As a result, a large area can be covered with a small amountof binders. Alternatively, the surface can be covered with a smallamount of binders, so that the proportion of an area in contact with thefilm-like binders with respect to the entire surface can be increased.The binders have low electric conductivity and thus might increase theresistance of the electrode when they cohere. Uniform dispersion of thebinders can inhibit cohesion of the binders, so that a favorableelectrode with high electric conductivity can be fabricated.

The current collector 101 may be subjected to surface treatment Examplesof such surface treatment include corona discharge treatment, plasmatreatment, and undercoat treatment. The surface treatment can increasethe wettability of the current collector 101 with respect to the slurry.In addition, the adhesion between the current collector 101 and theactive material layer 102 can be increased.

Here, the “undercoat” refers to a film formed over a current collectorbefore application of slurry onto the current collector for the purposeof reducing the interface resistance between an active material layerand the current collector or increasing the adhesion between the activematerial layer and the current collector. Note that the undercoat is notnecessarily formed in a film shape, and may be formed in an islandshape. In addition, the undercoat may serve as an active material tohave capacity. For the undercoat, a carbon material can be used, forexample. Examples of the carbon material include graphite, carbon blacksuch as acetylene black and ketjen black, and a carbon nanotube.

Then, the slurry is applied to the current collector 101.

For the application, a slot the method, a gravure method, a blademethod, or combination of any of them can be used. Furthermore, acontinuous coater or the like may be used for the application.

Then, the solvent of the slurry is volatilized to form the activematerial layer 102. The steps for volatilizing the solvent of the slurryare as follows, for example. Heat treatment is performed using a hotplate at 30° C. or higher and 70° C. or lower in an air atmosphere forlonger than or equal to 10 minutes, and then, for example, another heattreatment is performed at room temperature or higher and 100° C. orlower in a reduced-pressure environment for longer than or equal to 1hour and shorter than or equal to 10 hours.

Alternatively, heat treatment may be performed using a drying furnace orthe like. In the case of using a drying furnace, the heat treatment isperformed at 30° C. or higher and 120° C. or lower for longer than orequal to 30 seconds and shorter than or equal to 20 minutes, forexample. The temperature may be increased in stages. For example, afterheat treatment is performed at 60° C. or lower for shorter than or equalto 10 minutes, another heat treatment may further be performed at higherthan or equal to 63° C. for longer than or equal to 1 minute.

The thickness of the active material layer 102 formed through the abovesteps is, for example, preferably greater than or equal to 5 μm and lessthan or equal to 300 μm, more preferably greater than or equal to 10 μmand less than or equal to 130 μm. Furthermore, the amount of the activematerial in the active material layer 102 is, for example, preferablygreater than or equal to 2 mg/cm² and less than or equal to 30 mg/cm².

The active material layer 102 may be formed over only one surface of thecurrent collector 101, or the active material layers 102 may be formedsuch that the current collector 101 is sandwiched therebetween.Alternatively, the active material layers 102 may be formed such thatpart of the current collector 101 is sandwiched therebetween.

Note that the active material layer 102 may be predoped. There is noparticular limitation on the method for predoping the active materiallayer 102. For example, the active material layer 102 may be predopedelectrochemically. For example, before the battery is assembled, theactive material layer 102 can be predoped with lithium in anelectrolytic solution described later with the use of a lithium metal asa counter electrode.

Embodiment 2

Described in this embodiment will be an example of a power storagedevice using the electrode of one embodiment of the present invention.

Note that the power storage device in this specification and the likeindicates all elements and devices that have the function of storingelectric power. For example, a storage battery such as a lithium-ionsecondary battery, a lithium-ion capacitor, and an electric double layercapacitor are included in the category of the power storage device.

[Thin Storage Battery]

FIG. 2A illustrates a thin storage battery as an example of a storagedevice. When a flexible thin storage battery is used in an electronicdevice at least part of which is flexible, the storage battery can bebent as the electronic device is bent.

FIG. 2 is an external view of a thin storage battery 500. FIG. 3A is across-sectional view along dashed-dotted line A1-A2 in FIG. 2 , and FIG.3B is a cross-sectional view along dashed-dotted line B1-B2 in FIG. 2 .The thin storage battery 500 includes a positive electrode 503 includinga positive electrode current collector 501 and a positive electrodeactive material layer 502, a negative electrode 506 including a negativeelectrode current collector 504 and a negative electrode active materiallayer 505, a separator 507, an electrolytic solution 508, and anexterior body 509. The separator 507 is provided between the positiveelectrode 503 and the negative electrode 506 in the exterior body 509.The electrolytic solution 508 is included in the exterior body 509.

As at least one of the positive electrode 503 and the negative electrode506, the electrode of one embodiment of the present invention is used.The electrode of one embodiment of the present invention may be used asboth the positive electrode 503 and the negative electrode 506.

In the case where both the positive electrode 503 and the negativeelectrode 506 are the electrodes of embodiments of the presentinvention, the decomposition of the electrolytic solution caused by anegative electrode reaction (e.g., mainly, oxidative decomposition) andthe decomposition of the electrolytic solution caused by a positiveelectrode reaction (e.g., mainly, reductive decomposition) can beinhibited. Thus, it is possible that a storage battery having excellentproperties can be fabricated even with the use of an electrolyticsolution having a narrower potential window than a conventionalelectrolytic solution. In other words, the electrolytic solution used inthe storage battery can be selected from a wide range of alternatives.For example, a safer electrolytic solution such as a nonflammableelectrolytic solution to which fluorine is added might have lowresistance to oxidation; however, even in the case where such anelectrolytic solution is selected, a decrease in capacity by charge anddischarge can be inhibited, so that more excellent characteristics canbe achieved.

First, the structure of the negative electrode 506 will be described.The electrode of one embodiment of the present invention is preferablyused as the negative electrode 506. Note that the electrode of oneembodiment of the present invention may be used as the positiveelectrode 503. Here, an example of using the electrode 100 described inEmbodiment 1 as the negative electrode 506 will be described.

The negative electrode active material and the binder that are describedin Embodiment 1 are used for the negative electrode 506. The negativeelectrode 506 preferably includes a first binder and a second binder.Water-soluble polymers are preferably used as the first binder. Thebinder described in Embodiment 1 is used as the second binder; a binderhaving a styrene monomer or a butadiene monomer is preferably used.Furthermore, the conductive additive described in Embodiment 1 may beused for the negative electrode 506.

The negative electrode 506 is fabricated by the method described inEmbodiment 1.

Next, a structure of the positive electrode 503 will be described.

For the positive electrode current collector SOI, any of the examples ofthe materials for the current collector 101 listed above can be selectedto be used.

The positive electrode active material layer 502 includes a positiveelectrode active material. As described above, an “active material”refers only to a material that relates to insertion and extraction ofions functioning as carriers. In this specification and the like,however, a layer including a conductive additive, a binder, or the likeas well as a material that is actually a “active material” is alsoreferred to as an active material layer.

As a positive electrode active material, the positive electrode activematerial described in Embodiment 1 can be used.

The positive electrode active material layer 502 may further include aconductive additive. As the conductive additive, any of the materialsfor the conductive additive described in Embodiment 1 can be used, forexample.

The positive electrode active material layer 502 may further include abinder. As the binder, the binder described in Embodiment 1 is used, forexample.

Here, the positive electrode active material layer 502 preferablycontains graphene. Graphene is capable of making low-resistance surfacecontact and has extremely high conductivity even with a small thickness.Therefore, even a small amount of graphene can efficiently form aconductive path in an active material layer.

Here, for example, lithium-containing complex phosphate with an olivinecrystal structure used for the positive electrode active material has aone-dimensional lithium diffusion path, so that lithium diffusion isslow. The size of a primary particle of the active material is thuspreferably reduced to increase the charge and discharge rate.Furthermore, the specific surface area of the active material ispreferably increased.

In the case where such an active material with a small average particlesize (e.g., 1 μm or less) is used, the specific surface area of theactive material is large and thus more conductive paths for the activematerial particles are needed. In such a case, it is particularlypreferred that graphene with extremely high conductivity that canefficiently form a conductive path even in a small amount be used.

Next, a method for fabricating the positive electrode 503 will bedescribed.

FIG. 4A is a longitudinal sectional view of the positive electrodeactive material layer 502. The positive electrode active material layer502 includes positive electrode active material particles 522, grapheneflakes 521 as a conductive additive, and a binder (not illustrated).

The longitudinal section of the positive electrode active material layer502 in FIG. 4A shows substantially uniform dispersion of the grapheneflakes 521 in the positive electrode active material layer 502. Thegraphene flakes 521 are schematically shown by thick lines in FIG. 4Abut are actually thin films each having a thickness corresponding to thethickness of a single layer or a multi-layer of carbon molecules. Theplurality of graphene flakes 521 are formed in such a way as to wrap,coat, or adhere to the surfaces of the plurality of positive electrodeactive material particles 522, so that the graphene flakes 521 makesurface contact with the positive electrode active material particles522. Furthermore, the graphene flakes 521 are also in surface contactwith each other; consequently, the plurality of graphene flakes 521 forma three-dimensional network for electric conduction.

This is because graphene oxide with extremely high dispersibility in apolar solvent is used for the formation of the graphene flakes 521. Thesolvent is removed by volatilization from a dispersion medium in whichgraphene oxide is uniformly dispersed, and the graphene oxide is reducedto graphene; hence, the graphene flakes 521 remaining in the positiveelectrode active material layer 502 partly overlap with each other andare dispersed such that surface contact is made, thereby forming anelectrical conduction path.

Unlike a conventional conductive additive in the form of particles, suchas acetylene black, which makes point contact with an active material,the graphene flake 521 is capable of making low-resistance surfacecontact; accordingly, the electrical conduction between the positiveelectrode active material particles 522 and the graphene flakes 521 canbe improved without an increase in the amount of a conductive additive.Thus, the proportion of the positive electrode active material particles522 in the positive electrode active material layer 502 can beincreased. This can increase the discharge capacity of a storagebattery.

Next, an example of a method for fabricating a positive electrode inwhich graphene is used as a conductive additive will be described.First, an active material, a binder, and graphene oxide are prepared.

The graphene oxide is a raw material of the graphene flakes 521 thatserves as a conductive additive later. Graphene oxide can be formed byvarious synthesis methods such as a Hummers method, a modified Hummersmethod, and oxidation of graphite. Note that a method for fabricating astorage battery electrode of the present invention is not limited by thedegree of separation of graphene oxide.

For example, the Hummers method is a method for forming graphite oxideby oxidizing graphite such as flake graphite. The obtained graphiteoxide is graphite that is oxidized in places and thus to which afunctional group such as a carbonyl group, a carboxyl group, or ahydroxyl group is bonded. In the graphite oxide, the crystallinity ofthe graphite is lost and the distance between layers is increased.Therefore, the layers can be easily separated by ultrasonic treatment orthe like to obtain graphene oxide.

The length of one side (also referred to as a flake size) of thegraphene oxide is greater than or equal to 50 nm and less than or equalto 100 μm, preferably greater than or equal to 800 run and less than orequal to 20 μm. Particularly in the case where the flake size is smallerthan the average particle size of the positive electrode active materialparticles 203, the surface contact with a plurality of the positiveelectrode active material particles 522 and connection between grapheneflakes become difficult, resulting in difficulty in improving theelectrical conductivity of the positive electrode active material layer502.

Positive electrode slurry is formed by adding a solvent to such grapheneoxide, an active material, and a binder. As the solvent, water or apolar organic solvent such as N-methylpyrrolidone (NMP) ordimethylformamide can be used.

With the use of the active material layer including the active materialparticles, graphene, and the binder in the above-described manner, agraphene flake and part of the alloy-based material particles have asurface contact so that the flake surrounds the particles, and grapheneflakes also have surface contact to overlap with each other; thus, anextensive network of three-dimensional electric conduction paths isestablished in the active material layer. For this reason, it ispossible to form an active material layer with higher electricconductivity than a negative electrode active material layer includingacetylene black (AB) particles or ketjen black (KB) particles, which areconventionally used as a conductive additive and have an electricalpoint contact with an active material.

Furthermore, graphene is preferably used because even in the case ofusing, for example, an active material with a small particle size, theconductive path can be maintained even after charges and discharges arerepeatedly performed. Thus, favorable cycle characteristics can beachieved. A material that can have a surface contact with an activematerial, such as graphene, is preferably used in a storage batteryprovided in an electronic device that can be repeatedly folded, in whichcase the contact between the active material and a conductive additivecan be maintained even when the electronic device is folded. Graphene ispreferably used because it is flexible and thus allows a storage batteryto flexibly change its form when the storage battery is changed in formby being folded and can prevent breakage of the conductive additive whenthe storage battery is changed in form.

Graphene flakes are bonded to each other to form net-like graphene(hereinafter referred to as a graphene net). The graphene net coveringthe active material can function as a binder for binding particles. Theamount of a binder can thus be reduced, or the binder does not have tobe used. This can increase the proportion of the active material in theelectrode volume or weight. That is to say, the capacity of the powerstorage device can be increased.

In the case where the positive electrode active material layer 502includes a binder, the binder described in Embodiment 1 is used, forexample. One example is PVDF, which has high resistance to oxidation andis stable even in the case where foe battery reaction potential of thepositive electrode is high. Another example is water-soluble polymers,which have high dispensability and can be evenly dispersed with smallactive material particles. Thus, water-soluble polymers can functioneven in a smaller amount. A film containing water-soluble polymers thatcovers or is in contact with the surface of an active material caninhibit the decomposition of an electrolytic solution.

Note that the amount of graphene oxide is set to 0.1 wt % to 10 wt %inclusive, preferably 0.1 wt % to 5 wt % inclusive, more preferably 0.2wt % to 1 wt % inclusive with respect to the total weight of the mixtureof the graphene oxide, the positive electrode active material, theconductive additive, and the binder. In contrast, graphene obtainedafter the positive electrode slurry is applied to a current collectorand reduction is performed is included at least at 0.05 wt % to 5 wt %inclusive, preferably 0.05 wt % to 2.5 wt % inclusive, more preferably0.1 wt % to 0.5 wt % inclusive with respect to the total weight of thepositive electrode active material layer. This is because the weight ofgraphene obtained by reducing graphene oxide is approximately half thatof the graphene oxide.

Note that a polar solvent may be further added after the mixing so thatfoe viscosity of the mixture can be adjusted. Mixing and addition of apolar solvent may be repeated more than once.

Subsequently, the positive electrode slurry is applied to the currentcollector.

Then, the solvent is volatilized from the slurry applied to the positiveelectrode current collector 501 by a method such as ventilation dryingor reduced pressure (vacuum) drying, whereby the positive electrodeactive material layer 502 is formed. The drying step is preferablyperformed using, for example, a hot wind at a temperature higher than orequal to 50° C. and lower than or equal to 160° C. There is noparticular limitation on the atmosphere.

Note that the positive electrode active material layer 502 may be formedover only one surface of the positive electrode current collector 501,or the positive electrode active material layers 502 may be formed suchthat the positive electrode current collector 501 is sandwichedtherebetween. Alternatively, the positive electrode active materiallayers 502 may be formed such that part of the positive electrodecurrent collector 501 is sandwiched therebetween.

The positive electrode current collector 501 may be subjected to surfacetreatment. Examples of such surface treatment include corona dischargetreatment, plasma treatment, and undercoat treatment. The surfacetreatment increases the wettability of the positive electrode currentcollector SOI to the positive electrode slurry. In addition, theadhesion between the positive electrode current collector 501 and thepositive electrode active material layer 502 can be increased.

The positive electrode active material layer 502 may be pressed by acompression method such as a roll press method or a flat plate pressmethod so as to be consolidated.

Then, graphene oxide is preferably reduced by heat treatment or with theuse of a reducing agent, for example.

An example of a reducing method using a reducing agent will be describedbelow. First, a reaction is caused in a solvent containing a reducingagent. Through this step, the graphene oxide contained in the activematerial layer is reduced to form the graphene flakes 521. Note that itis possible that oxygen in the graphene oxide is not necessarilyentirely released and may partly remain in the graphene. When thegraphene flakes 521 contain oxygen, the proportion of oxygen measured byXPS is higher than or equal to 2 at. % and lower than or equal to 20 at.%, preferably higher than or equal to 3 at. % and lower titan or equalto 15 at. %. This reduction treatment is preferably performed at higherthan or equal to room temperature and lower than or equal to 150° C.

Examples of the reducing agent include ascorbic acid, hydrazine,dimethyl hydrazine, hydroquinone, sodium boron hydride (NaBHf), tetrabutyl ammonium bromide (TBAB), LiAlH₄, ethylene glycol, polyethyleneglycol, N,N-diethylhydroxylamine, and a derivative thereof.

A polar solvent can be used as the solvent. Any material can be used forthe polar solvent as long as it can dissolve the reducing agent.Examples of the material of tire polar solvent include water, methanol,ethanol, acetone, tetrahydrofuran (THF), dimethylformamide (DMF),N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and a mixedsolution of any two or more of the above.

After that, washing and drying are performed. The drying is preferablyperformed under a reduced pressure (in vacuum) or in a reductionatmosphere. This drying step is preferably performed, for example, invacuum at a temperature higher than or equal to 50° C. and lower than orequal to 160° C. for longer than or equal to 10 minutes and shorter thanor equal to 48 hours. The drying allows evaporation, volatilization, orremoval of the polar solvent and moisture in the positive electrodeactive material layer 502. The drying may be followed by pressing.

Alternatively, the drying may be performed using a drying furnace or thelike. In the case of using a drying furnace, the drying is performed at30° C. to 200° C. inclusive for longer than or equal to 30 seconds andshorter than or equal to 20 minutes, for example. The temperature may beincreased in stages.

Note that heating can facilitate the reduction reaction caused using thereducing agent. After drying following the chemical reduction, heatingmay further be performed.

In the case of not performing reduction with the use of a reducingagent, reduction can be performed by heat treatment. For example,reduction by heat treatment can be performed under a reduced pressure(in vacuum) at a temperature higher than or equal to 150° C. for longerthan or equal to 0.5 hours and shorter than or equal to 30 hours.

Through the above steps, the positive electrode active material layer502 in which the positive electrode active material particles 522 andthe graphene flakes 521 are uniformly dispersed can be formed.

Here, reduction is preferably performed on an electrode using grapheneoxide. It is more preferred that reduction be performed in such a mannerthat chemical reduction and thermal reduction are performed in thisorder. In thermal reduction, oxygen atoms are released in the form of,for example, carbon dioxide. In contrast, in chemical reduction,reduction is performed using a chemical reaction, whereby the proportionof carbon atoms that form an sp² bond of graphene can be high.Furthermore, thermal reduction is preferably performed after chemicalreduction, in which case the conductivity of formed graphene can befurther increased.

The thickness of the positive electrode active material layer 502 formedin the above-described manner is preferably greater than or equal to 5μm and less than or equal to 300 μm, more preferably greater than orequal to 10 μm and less than or equal to 150 μm, for example. The amountof the active material in the active material layer 102 is preferablygreater than or equal to 2 mg/cm² and less than or equal to 50 mg/cm²,for example.

The use of LiFePO₄ for the positive electrode allows fabrication of ahighly safe storage battery that is stable to an external load such asovercharge. That is to say, according to one embodiment of the presentinvention, a long-life and highly safe storage battery can befabricated. Such a storage battery is particularly suitable for, forexample, a mobile device, a wearable device, and the like.

For example, in storage batteries provided in electronic devices thatcan be repeatedly folded, exterior bodies gradually deteriorate andcracks are likely to be caused in some cases as the electronic devicesare folded repeatedly. Furthermore, the contact between a surface of anactive material and the like and an electrolytic solution by charge anddischarge causes a decomposition reaction, which might generate a gas orthe like. When expanded because of generation of a gas, the exteriorbodies are more likely to be damaged as the electronic devices arefolded. Even in such a case, according to one embodiment of the presentinvention, for example, generation of a gas by charge and discharge canbe inhibited, and consequently, expansion, deformation, damage, and thelike of the exterior bodies can be suppressed. This reduces a load onfoe exterior body, which is preferable.

Furthermore, the electrode of one embodiment of the present inventionused as foe negative electrode can inhibit the decomposition of anelectrolytic solution and thus can also inhibit growth of a coatingfilm. The resistance of a negative electrode in which growth of acoating film is significant increases as the number of charge anddischarge cycles is increased. In such a negative electrode, lithiumdeposition might occur because of, for example, stress caused when anelectronic device is folded. For example, wrinkles and fold lines formedon an electrode when an electronic device is folded might formunevenness. In the electrode with such unevenness, lithium intercalationinto graphite particles proceeds at projections first (the depth ofcharge is larger than that at a region other than the projections, sothat lithium deposition might occur more easily at the projections. Theelectrode of one embodiment of the present invention used as thenegative electrode has durability to stress caused when an electronicdevice is folded, and thus can reduce the possibility of causing lithiumdeposition, for example.

Here, the ratio of the capacity of a positive electrode of a storagebattery to the capacity of a negative electrode of the storage batterywill be described. A variable R defined by Mathematical Formula 5 belowis the ratio of positive electrode capacity to negative electrodecapacity. Here, positive electrode capacity means the capacity of thepositive electrode of the storage battery, and negative electrodecapacity means the capacity of the negative electrode of the storagebattery.

                         [Mathematical  Formula  5]$R = {\frac{{Positive}\mspace{14mu}{electrode}\mspace{14mu}{capacity}}{{Negative}\mspace{14mu}{electrode}\mspace{14mu}{capacity}} \times {100\mspace{14mu}\lbrack\%\rbrack}}$

Here, the theoretical capacity or the like may be used for calculationof foe positive electrode capacity and the negative electrode capacity.Alternatively, capacity based on a measured value or the like may beused. For example, in the case where LiFePO₄ and graphite are used, thecapacity per unit weight of the active material of LiFePO₄ is 170 mAh/g,and that of graphite is 372 mAh/g.

In the case where the ratio R of the positive electrode capacity to thenegative electrode capacity is small, the depth of charge and dischargeof graphite is small, whereas in the case where the ratio R of thepositive electrode capacity to the negative electrode capacity is large,the depth of charge and discharge of graphite is large, so that changesof graphite due to expansion and contraction caused by charge anddischarge increase, which might decrease the strength of the electrode.Consequently, the reliability of the power storage device might bedecreased. For example, the capacity might be reduced through a largenumber of charge and discharge cycles.

In contrast, in the case where the ratio R of the positive electrodecapacity to the negative electrode capacity is large, the specificsurface area of graphite with respect to the capacity of the powerstorage device can be small, which can inhibit a decomposition reactionof an electrolytic solution that occurs on the surface of graphite.

The smaller the depth of charge and discharge of graphite is, thesmaller changes in volume due to expansion and contraction are. When thechange in volume is small, a coating film formed in the initial chargeis less likely to be damaged, for example. Furthermore, the loss of aconductive path of the electrode due to expansion and contraction, orthe like can presumably be suppressed. This leads to inhibition of areduction in capacity with increasing number of charge and dischargecycles.

In addition, for example, in the case where a storage battery providedin an electronic device can be repeatedly folded is expanded andcontracted by charge and discharge and deformed as the electronic deviceis folded, the storage battery deteriorates at high speed compared withthe case where folding is not performed. When the depth of charge anddischarge is rendered small to inhibit expansion and contraction,however, deterioration can probably be small.

Here, the ratio R of the positive electrode capacity to the negativeelectrode capacity is, for example, preferably 20% to 90% inclusive,more preferably 30% to 75% inclusive, still more preferably 35% to 50%inclusive.

The optimum value of the depth of charge and discharge of graphite ispreferably 20% to 90% inclusive, more preferably 30% to 75% inclusive,still more preferably 35% to 50% inclusive.

As a solvent of the electrolytic solution 508, an aprotic organicsolvent is preferably used. For example, one of ethylene carbonate (EC),propylene carbonate (PC), butylene carbonate, chloroethylene carbonate,vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate(DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methylformate, methyl acetate, methyl butyrate, 1,3-dioxane, 1,4-dioxane,dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyldiglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, andsultone can be used, or two or more of these solvents can be used in anappropriate combination in an appropriate ratio.

When a high-molecular material that undergoes gelation is used as asolvent of the electrolytic solution, safety against liquid leakage andthe like is improved. Furthermore, a secondary battery can be thinnerand more lightweight. Typical examples of the high-molecular materialthat undergoes gelation include a silicone gel, an acrylic gel, anacrylonitrile gel, a polyethylene oxide-based gel, a polypropyleneoxide-based gel, a fluorine-based polymer gel, and the like.

Alternatively, the use of one or more kinds of ionic liquids (roomtemperature molten salts) which have features of non-flammability andnon-volatility as a solvent of the electrolytic solution can prevent apower storage device from exploding or catching fire even when a powerstorage device internally shorts out or the internal temperatureincreases owing to overcharging or the like. An ionic liquid contains acation and an anion. The ionic liquid of one embodiment of the presentinvention contains an organic cation and an anion. Examples of theorganic cation used for the electrolytic solution include aliphaticonium cations such as a quaternary ammonium cation, a tertiary sulfoniumcation, and a quaternary phosphonium cation, and aromatic cations suchas an imidazolium cation and a pyridinium cation. Examples of the anionused for the electrolytic solution include a monovalent amide-basedanion, a monovalent methide-based anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, a tetrafluoroborate anion, aperfluoroalkylborate anion, a hexafluorophosphate anion, and aperfluoroalkylphosphate anion.

In the case of using lithium ions as carriers, as an electrolytedissolved in the above-described solvent, one of lithium salts such asLiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄,Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂)(CF₃SO₂), and LiN(C₂F₅SO₂)₂ canbe used, or two or more of these lithium salts can be used in anappropriate combination in an appropriate ratio.

The electrolytic solution used for a power storage device is preferablyhighly purified and contains a small amount of dust particles andelements other than the constituent elements of the electrolyticsolution (hereinafter, also simply referred to as impurities).Specifically, the weight ratio of impurities to the electrolyticsolution is less than or equal to 1%, preferably less than or equal to0.1%, and more preferably less than or equal to 0.01%.

Furthermore, an additive agent such as vinylene carbonate, propanesultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC),or LiBOB may be added to the electrolytic solution. The concentration ofsuch an additive agent in the whole solvent is, for example, higher thanor equal to 0.1 wt % and lower than or equal to 5 wt %.

Alternatively, a gelled electrolyte obtained in such a manner that apolymer is swelled with an electrolytic solution may be used. Examplesof the gelled electrolyte (polymer-gel electrolyte) include a hostpolymer that is used as a support and contains the electrolytic solutiondescribed above.

Examples of host polymers include a polymer having a polyalkylene oxidestructure, such as polyethylene oxide (PEO); PVDF; polyacrylonitrile;and a copolymer containing any of them. For example, PVDF-HFP, which isa copolymer of PVDF and hexafluoropropylene (HFP) can be used. Theformed polymer may be porous.

Instead of the electrolytic solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including amacromolecular material such as a polyethylene oxide (PEO)-basedmacromolecular material may alternatively be used. When the solidelectrolyte is used, a separator and a spacer are not necessary.Furthermore, the battery can be entirely solidified; therefore, there isno possibility of liquid leakage and thus the safety of the battery isdramatically increased.

As the separator 507, paper; nonwoven fabric; glass fiber; ceramics;synthetic fiber containing polyamide, vinylon (polyvinyl alcohol-basedfiber), polyester, acrylic, polyolefin, or polyurethane; or the like canbe used.

The separator 507 is preferably formed to have a bag-like shape tosurround one of the positive electrode 503 and the negative electrode506. For example, as illustrated in FIG. 5A, the separator 507 is foldedin two so that the positive electrode 503 is sandwiched, and sealed witha sealing member 514 in a region outside the region overlapping with thepositive electrode 503; thus, the positive electrode 503 can be reliablysupported inside the separator 507. Then, as illustrated in FIG. 5B, thepositive electrodes 503 surrounded by the separators 507 and thenegative electrodes 506 are alternately stacked and provided in theexterior body 509, whereby the thin storage battery 500 can be formed.

Here, the operation of storage batteries will be described.

The operating principle of secondary batteries will be described using alithium-ion secondary battery as an example. Here, for example, LiFePO₄and graphite are used as a positive electrode active material and anegative electrode active material, respectively.

FIG. 41 illustrates connections between a lithium-ion secondary battery1101 and a charger 1102 when the lithium-ion secondary battery ischarged. In the case of charging the lithium-ion secondary battery, areaction expressed by Formula 6 occurs in a positive electrode.LiFePO₄→FePO₄+Li⁺ +e ⁻  [Formula 6]

In addition, a reaction expressed by Formula 7 occurs in a negativeelectrode.xC+Li⁺ +e ⁻→LiC_(x) x≥6  [Formula 7]

FIG. 42 illustrates connections between the lithium-ion secondarybattery 1101 and a load 1103 when the lithium-ion secondary battery isdischarged. In the case of discharging the lithium-ion secondarybattery, a reaction expressed by Formula 8 occurs in the positiveelectrode.FePO₄+Li⁺ +e ⁻→LiFePO₄  [Formula 8]

In addition, a reaction expressed by Formula 9 occurs in the negativeelectrode.LiC_(x) →xC+Li⁺ +e ⁻ x≥6  [Formula 9]

Here, an electrode of one embodiment of the present invention will bedescribed with reference to FIGS. 4B and 4C. FIG. 4B illustrates thestate where a film-like binder 2104 is in contact with a surface of anactive material 2103. In an electrolytic solution, solvent molecules2106 are solvated with a cation 2105 that contributes to a batteryreaction (e.g., a lithium ion in a lithium-ion secondary battery).Although FIG. 4B illustrates the state where two solvent molecules aresolvated for simplicity, it is needless to say that more than twosolvent molecules may be solvated. The solvent molecules 2106 aredesolvated from the cation 2105 at the surface of the binder 2104 or thesurface of the active material 2103, and the cation 2105 is insertedinto the active material 2103 through the binder 2104 or from thesurface of the active material. Although not illustrated, release of thecation 2105 from the active material 2103 also occurs as a counterreaction.

Here, a coating film 2107 is formed on the surface of the activematerial mainly in the initial charge. It is believed that the coatingfilm 2107 is also grown in the initial discharge and the second andsubsequent charge and discharge cycles. The coating film 2107 is formedin such a manner that a solvent or a salt in the electrolytic solutionis decomposed at a potential at which a battery reaction is caused andthe decomposition product is deposited. Here, a coating film ispresumably not formed on the surface of the binder 2104, which functionsas a passivation film, or a coating film is presumably thinner on thesurface of the binder 2104 than on a portion where the binder 2104 doesnot exist or a portion where the binder 2104 is thin. In other words,decomposition of the electrolytic solution can be more inhibited on thesurface of the binder 2104 than on a portion where the binder 2104 doesnot exist or a portion where the binder 2104 is thin.

Here, decomposition of the electrolytic solution is caused by areduction reaction, an oxidation reaction, or the like, so that chargeis consumed. This causes the irreversible capacity of the battery.

Next, aging after fabrication of a storage battery will be described.Aging is preferably performed after fabrication of a storage battery.The aging can be performed under the following conditions, for example.Charge is performed at a rate of 0.001 C or more and 0.2 C or less at atemperature higher than or equal to room temperature and lower than orequal to 50° C. In the case where an electrolytic solution is decomposedand a gas is generated and accumulated in the storage battery, theelectrolytic solution cannot be in contact with a surface of theelectrode in some regions. That is to say, an effectual reaction area ofthe electrode is reduced and effectual current density is increased.

When the current density is extremely high, a voltage drop occursdepending on the resistance of the electrode, lithium is intercalatedinto graphite and lithium is deposited on the surface of graphite. Thelithium deposition might reduce capacity. For example, if a coating filmor the like is grown on the surface after lithium deposition, lithiumdeposited on the surface cannot be dissolved again. This lithium cannotcontribute to capacity. In addition, when deposited lithium isphysically collapsed and conduction with the electrode is lost, thelithium also cannot contribute to capacity. Therefore, the gas ispreferably released before the potential of the electrode reaches thepotential of lithium because of a voltage drop.

After the release of the gas, the charging state may be maintained at atemperature higher than room temperature, preferably higher than orequal to 30° C. and lower than or equal to 60° C., more preferablyhigher than or equal to 35° C. and lower than or equal to 50° C. for,for example, 1 hour or more and 100 hours or less. In the initialcharge, an electrolytic solution decomposed on the surface forms acoating film. The formed coating film may thus be densified when thecharging state is held at a temperature higher than room temperatureafter the release of the gas, for example.

FIG. 6B illustrates an example where a current collector is welded to alead electrode, specifically, an example where positive electrodecurrent collectors 501 are welded to a positive electrode lead electrode510. The positive electrode current collectors 501 are welded to thepositive electrode lead electrode 510 in a welding region 512 byultrasonic welding or the like. The positive electrode current collector501 includes a bent portion 513 as illustrated in FIG. 6B, and it istherefore possible to relieve stress due to external force applied afterfabrication of the thin storage battery 500. Thus, the thin storagebattery 500 can have high reliability.

In the thin storage battery 500 illustrated in FIG. 2 and FIGS. 3A and3B, the positive electrode current collectors 501 and the negativeelectrode current collectors 504 are welded to the positive electrodelead electrode 510 and a negative electrode lead electrode 511,respectively, by ultrasonic welding so that part of the positiveelectrode lead electrode 510 and part of the negative electrode leadelectrode 511 are exposed to the outside. The positive electrode currentcollector 501 and the negative electrode current collector 504 candouble as terminals for electrical contact with the outside. In thatcase, the positive electrode current collector 501 and the negativeelectrode current collector 504 may be arranged so that part of thepositive electrode current collector 501 and part of the negativeelectrode current collector 504 are exposed to the outside the exteriorbody 509 without using lead electrodes.

Although the positive electrode lead electrode 510 and the negativeelectrode lead electrode 511 are provided on the same side in FIG. 2 ,the positive electrode lead electrode 510 and the negative electrodelead electrode 511 may be provided on different sides as illustrated inFIG. 7 . The lead electrodes of a storage battery of one embodiment ofthe present invention can be finely positioned as described above;therefore, the degree of freedom in design is high. Accordingly, aproduct including a storage battery of one embodiment of the presentinvention can have a high degree of freedom in design. Furthermore, anyield of products each including a storage battery of one embodiment ofthe present invention can be increased.

As the exterior body 509 in the thin storage battery 500, for example, afilm having a three-layer structure in which a highly flexible metalthin film of aluminum, stainless steel, copper, nickel, or the like isprovided over a film formed of a material such as polyethylene,polypropylene, polycarbonate, ionomer, or polyamide, and an insulatingsynthetic resin film of a polyamide-based resin, a polyester-basedresin, or the like is provided as the outer surface of the exterior bodyover the metal thin film can be used.

The example in FIG. 2 includes five positive electrode-negativeelectrode pairs (the positive and negative electrodes of each pair faceeach other). It is needless to say that the number of pairs ofelectrodes is not limited to five, and may be more than five or lessthan five. In the case of a large number of electrode layers, thestorage battery can have high capacity. In contrast, in the case ofusing a small number of electrode layers, the storage battery can have asmall thickness and high flexibility.

In the above structure, the exterior body 509 of the storage battery canchange its form with a radius of greater than or equal to 10 mm,preferably greater than or equal to 30 mm. One or two films are used asthe exterior body of the storage battery. In the case where the storagebattery has a layered structure, the storage battery has a cross sectionsandwiched by two curved surfaces of the films when it is bent.

Description is given of the radius of curvature of a surface withreference to FIGS. 8A to 8C. In FIG. 8A, on a plane 1701 along which acurved surface 1700 is cut, part of a curve 1702 of the curved surface1700 is approximate to an arc of a circle, and the radius of the circleis referred to as a radius 1703 of curvature and the center of thecircle is referred to as a center 1704 of curvature. FIG. 8B is a topview of the curved surface 1700. FIG. 8C is a cross-sectional view ofthe curved surface 1700 taken along the plane 1701. When a curvedsurface is cut by a plane, the radius of curvature of a curve in a crosssection differs depending on the angle between the curved surface andthe plane or on the cut position, and the smallest radius of curvatureis defined as the radius of curvature of a surface in this specificationand the like.

In the case of bending a secondary battery in which a component 1805including electrodes and an electrolytic solution is sandwiched betweentwo films as exterior bodies, a radius 1802 of curvature of a film 1801close to a center 1800 of curvature of the secondary battery is smallerthan a radius 1804 of curvature of a film 1803 far from the center 1800of curvature (FIG. 9A). When the secondary battery is curved and has anarc-shaped cross section, compressive stress is applied to a surface ofthe film on the side closer to the center 1800 of curvature and tensilestress is applied to a surface of the film on the side farther from thecenter 1800 of curvature (FIG. 9B). However, by forming a patternincluding projections or depressions on surfaces of the exterior bodies,the influence of a strain can be reduced to be acceptable even whencompressive stress and tensile stress are applied. For this reason, thesecondary battery can change its form such that the exterior body on theside closer to the center of curvature has a curvature radius greaterthan or equal to 10 mm, preferably greater than or equal to 30 mm

Note that the cross-sectional shape of the secondary battery is notlimited to a simple arc shape, and the cross section can be partlyarc-shaped; for example, a shape illustrated in FIG. 9C, a wavy shapeillustrated in FIG. 9D, or an S shape can be used. When the curvedsurface of the secondary battery has a shape with a plurality of centersof curvature, the secondary battery can change its form such that acurved surface with the smallest radius of curvature among radii ofcurvature with respect to the plurality of centers of curvature, whichis a surface of the exterior body on the side closer to the center ofcurvature, has a curvature radius greater than or equal to 10 mm,preferably greater than or equal to 30 mm.

[Coin-Type Storage Battery]

Next, an example of a coin-type storage battery will be described as anexample of a power storage device with reference to FIGS. 10A and 10B.FIG. 10A is an external view of a coin-type (single-layer flat type)storage battery, and FIG. 10B is a cross-sectional view thereof.

In a coin-type storage battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. Thedescription of the positive electrode active material layer 502 can bereferred to for the positive electrode active material layer 306.

A negative electrode 307 includes a negative electrode current collector308 and a negative electrode active material layer 309 provided incontact with the negative electrode current collector 308. Thedescription of the negative electrode active material layer 505 can bereferred to for the negative electrode active material layer 309. Thedescription of the separator 507 can be referred to for the separator310. The description of the electrolytic solution 508 can be referred tofor the electrolytic solution.

Note that only one surface of each of the positive electrode 304 and thenegative electrode 307 used for the coin-type storage battery 300 isprovided with an active material layer.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolytic solution,such as nickel, aluminum, or titanium, an alloy of such a metal, or analloy of such a metal and another metal (e.g., stainless steel or thelike) can be used. Alternatively, the positive electrode can 301 and thenegative electrode can 302 are preferably covered with nickel, aluminum,or the like in order to prevent corrosion due to the electrolyticsolution. The positive electrode can 301 and the negative electrode can302 are electrically connected to the positive electrode 304 and thenegative electrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolytic solution. Then, asillustrated in FIG. 10B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303interposed therebetween. In such a manner, the coin-type storage battery300 can be manufactured.

[Cylindrical Storage Battery]

Next, an example of a cylindrical storage battery will be described asan example of a power storage device with reference to FIGS. 11A and11B. As illustrated in FIG. 11A, a cylindrical storage battery 600includes a positive electrode cap (battery cap) 601 on the top surfaceand a battery can (outer can) 602 on the side surface and bottomsurface. The positive electrode cap 601 and the battery can 602 areinsulated from each other by a gasket (insulating gasket) 610.

FIG. 11B is a diagram schematically illustrating a cross section of thecylindrical storage battery. Inside the battery can 602 having a hollowcylindrical shape, a battery element in which a strip-like positiveelectrode 604 and a strip-like negative electrode 606 are wound with astripe-like separator 603 interposed therebetween is provided. Althoughnot illustrated, the battery element is wound around a center pin. Oneend of the battery can 602 is close and the other end thereof is open.For the battery can 602, a metal having a corrosion-resistant propertyto an electrolytic solution, such as nickel, aluminum, or titanium, analloy of such a metal, or an alloy of such a metal and another metal(e.g., stainless steel or the like) can be used. Alternatively, thebattery can 602 is preferably covered with nickel, aluminum, or the likein order to prevent corrosion due to the electrolytic solution. Insidethe battery can 602, the battery element in which the positiveelectrode, the negative electrode, and the separator are wound isprovided between a pair of insulating plates 608 and 609 which face eachother. Furthermore, a nonaqueous electrolytic solution (not illustrated)is injected inside the battery can 602 provided with the batteryelement. As the nonaqueous electrolytic solution, a nonaqueouselectrolytic solution that is similar to those of the coin-type storagebattery can be used.

The positive electrode 604 and the negative electrode 606 can be formedin a manner similar to that of the positive electrode and the negativeelectrode of the thin storage battery described above. Since thepositive electrode and the negative electrode of the cylindrical storagebattery are wound, active materials are preferably formed on both sidesof the current collectors. A positive electrode terminal (positiveelectrode current collecting lead) 603 is connected to the positiveelectrode 604, and a negative electrode terminal (negative electrodecurrent collecting lead) 607 is connected to the negative electrode 606.Both the positive electrode terminal 603 and the negative electrodeterminal 607 can be formed using a metal material such as aluminum. Thepositive electrode terminal 603 and the negative electrode terminal 607are resistance-welded to a safety valve mechanism 612 and the bottom ofthe battery can 602, respectively. The safety valve mechanism 612 iselectrically connected to the positive electrode cap 601 through apositive temperature coefficient (PTC) element 611. The safety valvemechanism 612 cuts off electrical connection between the positiveelectrode cap 601 and the positive electrode 604 when the internalpressure of the battery exceeds a predetermined threshold value. The PTCelement 611, which serves as a thermally sensitive resistor whoseresistance increases as temperature rises, limits foe amount of currentby increasing the resistance, in order to prevent abnormal heatgeneration. Note that barium titanate (BaTiO₃)-based semiconductorceramic can be used for the PTC element.

Note that in this embodiment, the coin-type storage battery, thecylindrical storage battery, and the thin storage battery are given asexamples of the storage battery; however, any of storage batteries witha variety of shapes, such as a sealed storage battery and a square-typestorage battery, can be used. Furthermore, a structure in which aplurality of positive electrodes, a plurality of negative electrodes,and a plurality of separators are stacked or wound may be employed. Forexample, FIGS. 12A to 12C, FIGS. 13A to 13C, FIGS. 14A and 14B, FIGS.15A1 to 15B2, and FIGS. 16A and 16B illustrate examples of other storagebatteries.

[Structural Example of Storage Battery]

FIGS. 12A to 12C and FIGS. 13A to 13C illustrate structural examples ofthin storage batteries. A wound body 993 illustrated in FIG. 12Aincludes a negative electrode 994, a positive electrode 995, and aseparator 996.

The wound body 993 is obtained by winding a sheet of a stack in whichthe negative electrode 994 overlaps with the positive electrode 995 withthe separator 996 therebetween. The wound body 993 is covered with arectangular sealed container or the like; thus, a rectangular secondarybattery is fabricated.

Note that the number of stacks each including the negative electrode994, the positive electrode 995, and the separator 996 is determined asappropriate depending on capacity and element volume which are required.The negative electrode 994 is connected to a negative electrode currentcollector (not illustrated) via one of a lead electrode 997 and a leadelectrode 998. The positive electrode 995 is connected to a positiveelectrode current collector (not illustrated) via the other of the leadelectrode 997 and the lead electrode 998.

In a storage battery 990 illustrated in FIGS. 12B and 12C, the woundbody 993 is packed in a space formed by bonding a film 981 and a film982 having a depressed portion that serve as exterior bodies bythermocompression bonding or the like. The wound body 993 includes thelead electrode 997 and the lead electrode 998, and is soaked in anelectrolytic solution inside a space surrounded by the film 981 and thefilm 982 having a depressed portion.

For the film 981 and the film 982 having a depressed portion, a metalmaterial such as aluminum or a resin material can be used, for example.With the use of a resin material for the film 981 and the film 982having a depressed portion, the film 981 and the film 982 having adepressed portion can be changed in their forms when external force isapplied; thus, a flexible storage battery can be fabricated.

Although FIOS. 12B and 12C illustrate an example where a space is formedby two films, the wound body 993 may be placed in a space formed bybending one film.

Furthermore, not only a thin storage battery but also an exterior bodyand a sealed container of the storage battery may have flexibility in apower storage device. In that case, a resin material or the like is usedfor the exterior body and the sealed container. Note that in the casewhere a resin material is used for the exterior body and the sealedcontainer, a conductive material is used for a portion connected to theoutside.

For example, FIGS. 13B and 13C illustrate another example of a flexiblethin storage battery. The wound body 993 illustrated in FIG. 13A is thesame as that illustrated in FIG. 12A, and the detailed descriptionthereof is omitted.

In the storage battery 990 illustrated in FIGS. 13B and 13C, the woundbody 993 is packed in an exterior body 991. The wound body 993 includesthe lead electrode 997 and the lead electrode 998, and is soaked in anelectrolytic solution inside a space surrounded by the exterior body 991and an exterior body 992. For example, a metal material such as aluminumor a resin material can be used for the exterior bodies 991 and 992.With the use of a resin material for the exterior bodies 991 and 992,the exterior bodies 991 and 992 can be changed in their forms whenexternal force is applied; thus, a flexible thin storage battery can befabricated.

[Structural Example of Power Storage System]

Structural examples of power storage systems will be described withreference to FIGS. 14A and 14B, FIGS. 15A1 to 15B2, and FIGS. 16A and16B. Here, a power storage system refers to, for example, a deviceincluding a power storage device.

FIGS. 14A and 14B are external views of a power storage system. Thepower storage system includes a circuit board 900 and a storage battery913. A label 910 is attached to the storage battery 913. As shown inFIG. 14B, the power storage system further includes a terminal 951, aterminal 952, an antenna 914, and an antenna 915.

The circuit board 900 includes terminals 911 and a circuit 912. Theterminals 911 are connected to the terminals 951 and 952, the antennas914 and 915, and the circuit 912. Note that a plurality of terminals 911serving as a control signal input terminal, a power supply terminal, andthe like may be provided.

The circuit 912 may be provided on the rear surface of the circuit board900. The shape of each of the antennas 914 and 915 is not limited to acoil shape and may be a linear shape or a plate shape. Further, a planarantenna, an aperture antenna, a traveling-wave antenna, an EH antenna, amagnetic-field antenna, or a dielectric antenna may be used.Alternatively, the antenna 914 or the antenna 915 may be a flat-plateconductor. The flat-plate conductor can serve as one of conductors forelectric field coupling. That is, the antenna 914 or the antenna 915 canserve as one of two conductors of a capacitor. Thus, electric power canbe transmitted and received not only by an electromagnetic field or amagnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The power storage system includes a layer 916 between the storagebattery 913 and the antennas 914 and 915. The layer 916 may have afunction of blocking an electromagnetic field by the storage battery913. As the layer 916, for example, a magnetic body can be used.

Note that the structure of the power storage system is not limited tothat shown in FIGS. 14A and 14B.

For example, as shown in FIGS. 15A1 and 15A2, two opposite surfaces ofthe storage battery 913 in FIGS. 14A and 14B may be provided withrespective antennas. FIG. 15A1 is an external view showing one side ofthe opposite surfaces, and FIG. 15A2 is an external view showing theother side of the opposite surfaces. For portions similar to those inFIGS. 14A and 14B, the description of the power storage systemillustrated in FIGS. 14A and 14B can be referred to as appropriate.

As illustrated in FIG. 15A1, the antenna 914 is provided on one of theopposite surfaces of the storage battery 913 with the layer 916interposed therebetween, and as illustrated in FIG. 15A2, the antenna915 is provided on the other of the opposite surfaces of the storagebattery 913 with a layer 917 interposed therebetween. The layer 917 mayhave a function of preventing an adverse effect on an electromagneticfield by the storage battery 913. As the layer 917, for example, amagnetic body can be used.

With the above structure, both of the antennas 914 and 915 can beincreased in size.

Alternatively, as illustrated in FIGS. 15B1 and 15B2, two oppositesurfaces of the storage battery 913 in FIGS. 14A and 14B may be providedwith different types of antennas. FIG. 15B1 is an external view showingone side of the opposite surfaces, and FIG. 15B2 is an external viewshowing the other side of the opposite surfaces. For portions similar tothose in FIGS. 14A and 14B, the description of the power storage systemillustrated in FIGS. 14A and 14B can be referred to as appropriate.

As illustrated in FIG. 15B1, the antenna 914 is provided on one of theopposite surfaces of the storage battery 913 with the layer 916interposed therebetween, and as illustrated in FIG. 15B2, an antenna 918is provided on the other of the opposite surfaces of the storage battery913 with the layer 917 interposed therebetween. The antenna 918 has afunction of communicating data with an external device, for example. Anantenna with a shape that can be applied to the antennas 914 and 915,for example, can be used as the antenna 918. As a system forcommunication using the antenna 918 between the power storage system andanother device, a response method that can be used between the powerstorage system and another device, such as NFC, can be employed.

Alternatively, as illustrated in FIG. 16A, the storage battery 913 inFIGS. 14A and 14B may be provided with a display device 920. The displaydevice 920 is electrically connected to the terminal 911 via a terminal919. It is possible that the label 910 is not provided in a portionwhere the display device 920 is provided. For portions similar to thosein FIGS. 14A and 14B, the description of the power storage systemillustrated in FIGS. 14A and 14B can be referred to as appropriate.

The display device 920 can display, for example, an image showingwhether charge is being carried out, an image showing the amount ofstored power, or the like. As the display device 920, electronic paper,a liquid crystal display device, an electroluminescent (EL) displaydevice, or the like can be used. For example, the use of electronicpaper can reduce power consumption of the display device 920.191

Alternatively, as illustrated in FIG. 16B, the storage battery 913illustrated in FIGS. 14A and 14B may be provided with a sensor 921. Thesensor 921 is electrically connected to the terminal 911 via a terminal922. For portions similar to those in FIGS. 14A and 14B, the descriptionof the power storage system illustrated in FIGS. 14A and 14B can bereferred to as appropriate.

As the sensor 921, a sensor that has a function of measuring, forexample, force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,electric current, voltage, electric power, radiation, flow rate,humidity, gradient, oscillation, odor, or infrared rays can be used.With the sensor 921, for example, data on an environment (e.g.,temperature) where the power storage system is placed can be determinedand stored in a memory inside the circuit 912.

The electrode of one embodiment of the present invention is used in thestorage battery and the power storage system that are described in thisembodiment. The capacity of the storage battery and the power storagesystem can thus be high. Furthermore, energy density can be high.Moreover, reliability can be high, and life can be long.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 3

In this embodiment, an example of an electronic device including aflexible power storage device will be described.

FIGS. 17A to 17G illustrate examples of electronic devices including theflexible storage batteries described in Embodiment 2. Examples ofelectronic devices each including a flexible power storage deviceinclude television devices (also referred to as televisions ortelevision receivers), monitors of computers or the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as mobile phones or mobile phonedevices), portable game machines, portable information terminals, audioreproducing devices, and large game machines such as pachinko machines.

In addition, a flexible power storage device can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of a car.

FIG. 17A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a power storage device 7407.

FIG. 17B illustrates the mobile phone 7400 that is bent. When the wholemobile phone 7400 is bent by the external force, the power storagedevice 7407 included in the mobile phone 7400 is also bent. FIG. 17Cillustrates the bent power storage device 7407. The power storage device7407 is a thin storage battery. The power storage device 7407 is fixedin a state of being bent. Note that the power storage device 7407includes a lead electrode 7408 electrically connected to a currentcollector 7409.

FIG. 17D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a power storage device 7104. FIG. 17Eillustrates the bent power storage device 7104. When the display deviceis worn on a user's arm while the power storage device 7104 is beat, thehousing changes its form and the curvature of a part or the whole of thepower storage device 7104 is changed. Note that the radius of curvatureof a curve at a point refers to the radius of the circular arc that bestapproximates the curve at that point. The reciprocal of the radius ofcurvature is curvature. Specifically, a part or the whole of the housingor the main surface of the power storage device 7104 is changed in therange of radius of curvature from 40 mm to 150 mm. When the radius ofcurvature at the main surface of the power storage device 7104 isgreater than or equal to 40 mm and less than or equal to 150 mm, thereliability can be kept high.

FIG. 17F illustrates an example of a watch-type portable informationterminal. A portable information terminal 7200 includes a housing 7201,a display portion 7202, a band 7203, a buckle 7204, an operation button7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting texts, music reproduction, Internet communication, and acomputer game.

The display surface of the display portion 7202 is curved, and imagescan be displayed on the curved display surface. In addition, the displayportion 7202 includes a touch sensor, and operation can be performed bytouching the screen with a finger, a stylus, or the like. For example,by touching an icon 7207 displayed on the display portion 7202,application can be started.

With the operation button 7205, a variety of functions such as poweron/off, on/off of wireless communication, setting and cancellation of amanner mode, and setting and cancellation of a power saving mode can beperformed. For example, the functions of the operation button 7205 canbe set freely by setting the operation system incorporated in theportable information terminal 7200.

The portable information terminal 7200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 7200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible.

Moreover, the portable information terminal 7200 includes the inputoutput terminal 7206, and data can be directly transmitted to andreceived from another information terminal via a connector. In addition,charging via the input output terminal 7206 is possible. Note that thecharging operation may be performed by wireless power feeding withoutusing the input output terminal 7206.

The display portion 7202 of the portable information terminal 7200 isprovided with a power storage device including the electrode of oneembodiment of the present invention. For example, the power storagedevice 7104 illustrated in FIG. 17E that is in the state of being curvedcan be provided in the housing 7201. Alternatively, the power storagedevice 7104 illustrated in FIG. 17E can be provided in the band 7203such that it can be curved.

FIG. 17G illustrates an example of an armband display device. A displaydevice 7300 includes a display portion 7304 and the power storage deviceof one embodiment of the present invention. The display device 7300 caninclude a touch sensor in the display portion 7304 and can serve as aportable information terminal.

The display surface of the display portion 7304 is bent, and images canbe displayed on the bent display surface. A display state of the displaydevice 7300 can be changed by, for example, near field communication,which is a communication method based on an existing communicationstandard.

The display device 7300 includes an input output terminal, and data canbe directly transmitted to and received from another informationterminal via a connector. In addition, charging via the input outputterminal is possible. Note that the charging operation may be performedby wireless power feeding without using the input output terminal.

This embodiment can be implemented in appropriate combination with anyof foe other embodiments.

Embodiment 4

In this embodiment, examples of electronic devices that can includepower storage devices will be described.

FIGS. 18A and 18B illustrate an example of a tablet terminal that can befolded in half. A tablet terminal 9600 illustrated in FIGS. 18A and 18Bincludes a housing 9630 a, a housing 9630 b, a movable portion 9640connecting the housings 9630 a and 9630 b, a display portion 9631including a display portion 9631 a and a display portion 9631 b, adisplay mode changing switch 9626, a power switch 9627, a power savingmode changing switch 9625, a fastener 9629, and an operation switch9628. FIG. 18A illustrates the tablet terminal 9600 that is opened, andFIG. 18B illustrates the tablet terminal 9600 that is closed.

The tablet terminal 9600 includes a power storage unit 9635 inside thehousings 9630 a and 9630 b. The power storage unit 9635 is providedacross the housings 9630 a and 9630 b, passing through the movableportion 9640.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that FIG. 18A shows, as an example, that half of thearea of the display portion 9631 a has only a display function and theother half of the area has a touch panel function. However, thestructure of the display portion 9631 a is not limited to this, and allthe area of the display portion 9631 a may have a touch panel function.For example, all the area of the display portion 9631 a can display akeyboard and serve as a touch panel while the display portion 9631 b canbe used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display mode changing switch 9626 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power saving mode changing switch 9625 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal 9600, which is measured with anoptical sensor incorporated in the tablet terminal 9600. In addition tothe optical sensor, other detecting devices such as sensors fordetermining inclination, such as a gyroscope or an acceleration sensor,may be incorporated in the tablet terminal.

Although the display portion 9631 a and the display portion 9631 b havethe same area in FIG. 18A, one embodiment of the present invention isnot limited to this example. The display portion 9631 a and the displayportion 9631 b may have different areas or different display quality.For example, one of the display portions 9631 a and 9631 b may displayhigher definition images than the other.

The tablet terminal is closed in FIG. 18B. The tablet terminal includesthe housing 9630, a solar cell 9633, and a charge and discharge controlcircuit 9634 including a DCDC converter 9636. The power storage unit ofone embodiment of the present invention is used as the power storageunit 9635.

The tablet terminal 9600 can be folded such that the housings 9630 a and9630 b overlap with each other when not in use. Thus, the displayportions 9631 a and 9631 b can be protected, which increases thedurability of the tablet terminal 9600. In addition, the power storageunit 9635 of one embodiment of the present invention has flexibility andcan be repeatedly bent without a significant decrease in charge anddischarge capacity. Thus, a highly reliable tablet terminal can beprovided.

The tablet terminal illustrated in FIGS. 18A and 18B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, or the time on the display portion, a touch-input function ofoperating or editing data displayed on the display portion by touchinput, a function of controlling processing by various kinds of software(programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processing portion, and the like. Note that the solarcell 9633 can be provided on one or both surfaces of the housing 9630and the power storage unit 9635 can be charged efficiently. The use of alithium-ion battery as the power storage unit 9635 brings an advantagesuch as reduction in size.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 18B will be described with reference to a blockdiagram in FIG. 18C. The solar cell 9633, the power storage unit 9635,the DCDC converter 9636, a converter 9637, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 18C, and the power storageunit 9635, the DCDC converter 9636, the converter 9637, and the switchesSW1 to SW3 correspond to the charge and discharge control circuit 9634in FIG. 18B.

First, an example of operation when electric power is generated by thesolar cell 9633 using external light will be described. The voltage ofelectric power generated by the solar cell is raised or lowered by theDCDC converter 9636 to a voltage for charging the power storage unit9635. When the display portion 9631 is operated with the electric powerfrom the solar cell 9633, the switch SW1 is turned on and the voltage ofthe electric power is raised or lowered by the converter 9637 to avoltage needed for operating the display portion 9631. When display onthe display portion 9631 is not performed, the switch SW1 is turned offand the switch SW2 is turned on, so that the power storage unit 9635 canbe charged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, one embodiment of the present invention isnot limited to this example. The power storage unit 9635 may be chargedusing another power generation means such as a piezoelectric element ora thermoelectric conversion element (Peltier element). For example, thepower storage unit 9635 may be charged with a non-contact powertransmission module capable of performing charging by transmitting andreceiving electric power wirelessly (without contact), or any of theother charge means used in combination.

FIG. 19 illustrates other examples of electronic devices. In FIG. 19 , adisplay device 8000 is an example of an electronic device including apower storage device 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, and the power storage device 8004.The power storage device 8004 of one embodiment of the present inventionis provided in the housing 8001. The display device 8000 can receiveelectric power from a commercial power supply. Alternatively, thedisplay device 8000 can use electric power stored in the power storagedevice 8004. Thus, the display device 8000 can be operated with the useof the power storage device 8004 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 19 , an installation lighting device 8100 is an example of anelectronic device including a power storage device 8103 of oneembodiment of the present invention. Specifically, the lighting device8100 includes a housing 8101, a light source 8102, and the power storagedevice 8103. Although FIG. 19 illustrates the case where the powerstorage device 8103 is provided in a ceiling 8104 on which the housing8101 and the light source 8102 are installed, the power storage device8103 may be provided in the housing 8101. The lighting device 8100 canreceive electric power from a commercial power supply. Alternatively,the lighting device 8100 can use electric power stored in the powerstorage device 8103. Thus, the lighting device 8100 can be operated withthe use of power storage device 8103 of one embodiment of foe presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 19 as an example, the power storagedevice of one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Alternatively, the power storage device of one embodiment of the presentinvention can be used in a tabletop lighting device or the like.

As the light source 8102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, andlight-emitting elements such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 19 , an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including apower storage device 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, and the power storage device 8203. Although FIG. 19illustrates the case where the power storage device 8203 is provided inthe indoor unit 8200, the power storage device 8203 may be provided inthe outdoor unit 8204. Alternatively, the power storage devices 8203 maybe provided in both the indoor unit 8200 and the outdoor unit 8204. Theair conditioner can receive electric power from a commercial powersupply. Alternatively, the air conditioner can use electric power storedin the power storage device 8203. Particularly in the case where foepower storage devices 8203 are provided in both the indoor unit 8200 andthe outdoor unit 8204, the air conditioner can be operated with the useof the power storage device 8203 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 19 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 19 , an electric refrigerator-freezer 8300 is an example of anelectronic device including a power storage device 8304 of oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 8300 includes a housing 8301, a door for arefrigerator 8302, a door for a freezer 8303, and the power storagedevice 8304. The power storage device 8304 is provided in the housing8301 in FIG. 19 . The electric refrigerator-freezer 8300 can receiveelectric power from a commercial power supply. Alternatively, theelectric refrigerator-freezer 8300 can use electric power stored in thepower storage device 8304. Thus, the electric refrigerator-freezer 8300can be operated with the use of the power storage device 8304 of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from a commercial powersupply due to power failure or the like.

Note that among the electronic devices described above, a high-frequencyheating apparatus such as a microwave oven and an electronic device suchas an electric rice cooker require high power in a short time. Thetripping of a breaker of a commercial power supply in use of anelectronic device can be prevented by using the power storage device ofone embodiment of the present invention as an auxiliary power supply forsupplying electric power which cannot be supplied enough by a commercialpower supply.

In addition, in a tune period when electronic devices are not used,particularly when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied from a commercial power supply source (such a proportionreferred to as a usage rate of electric power) is low, electric powercan be stored in the power storage device, whereby the usage rate ofelectric power can be reduced in a time period when the electronicdevices are used. For example, in the case of the electricrefrigerator-freezer 8300, electric power can be stored in the powerstorage device 8304 in night time when the temperature is low and foedoor for a refrigerator 8302 and the door for a freezer 8303 are notoften opened or closed. On the other hand, in daytime when thetemperature is high and the door for a refrigerator 8302 and the doorfor a freezer 8303 are frequently opened and closed, the power storagedevice 8304 is used as an auxiliary power supply; thus, the usage rateof electric power in daytime can be reduced.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 5

In this embodiment, examples of vehicles using power storage deviceswill be described.

The use of power storage devices in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIOS. 20A and 20B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8400 illustrated inFIG. 20A is an electric vehicle that runs on the power of an electricmotor 8206. Alternatively, the automobile 8400 is a hybrid electricvehicle capable of driving appropriately using either the electric motor8206 or the engine. One embodiment of the present invention can providea high-mileage vehicle. The automobile 8400 includes the power storagedevice. The power storage device is used not only for driving theelectric motor 8206, but also for supplying electric power to alight-emitting device such as a headlight 8401 or a room light (notillustrated).

The power storage device can also supply electric power to a displaydevice of a speedometer, a tachometer, or the like included in theautomobile 8400. Furthermore, the power storage device can supplyelectric power to a semiconductor device included in the automobile8400, such as a navigation system.

FIG. 20B illustrates an automobile 8500 including the power storagedevice. The automobile 8500 can be charged when the power storage deviceis supplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.20B, a power storage device 8024 included in the automobile 8500 ischarged with the use of a ground-based charging apparatus 8021 through acable 8022. In charging, a given method such as CHAdeMO (registeredtrademark) or Combined Charging System may be employed as a chargingmethod, the standard of a connector, or the like as appropriate. Theground-based charging apparatus 8021 may be a charging station providedin a commerce facility or a power source in a house. For example, withthe use of a plug-in technique, the power storage device 8024 includedin the automobile 8500 can be charged by being supplied with electricpower from outside. The charging can be performed by converting ACelectric power into DC electric power through a converter such as anAC-DC converter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween vehicles. Furthermore, a solar cell may be provided in theexterior of the automobile to charge the power storage device when theautomobile stops or moves. To supply electric power in such acontactless manner, an electromagnetic induction method or a magneticresonance method can be used.

According to one embodiment of the present invention, the power storagedevice can have improved cycle characteristics and reliability.Furthermore, according to one embodiment of the present invention, thepower storage device itself can be made more compact and lightweight asa result of improved characteristics of the power storage device. Thecompact and lightweight power storage device contributes to a reductionin the weight of a vehicle, and thus increases the driving distance.Furthermore, the power storage device included in the vehicle can beused as a power source for supplying electric power to products otherthan the vehicle. In such a case, the use of a commercial power sourcecan be avoided at peak time of electric power demand.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Example 1

In this example, the characteristics of electrodes each formed usinggraphite as an active material will be described.

[Fabrication of Electrodes]

Electrodes were each fabricated using graphite as an active material.Table 1 shows the measured values of the specific surface areas and theaverage particle sizes of graphites that were used.

TABLE 1 Specific surface Particle area size [m²/g] [μm] Graphite A 1.511 Graphite B 1.3 9 Graphite C 2.7 20 Graphite D 6.3 15

Coated electrodes were fabricated under the conditions of the kinds ofgraphite and the compositions that are shown in Table 2. As a conductiveadditive, VGCF (registered trademark)-H (manufactured by SHOWA DENKOK.K., the fiber diameter 150 nm, the specific surface area: 13 m²/g),which is vapor grown carbon fiber, was used. Here, “amount” refers tothe amount of an active material. Note that as Graphite D, materials ofdifferent two lots (“Lot1” and “Lot2”) were used.

TABLE 2 Condition Series Composition [wt %] Electrode A-1 Graphite ASeries 1 Graphite:CMC-Na:SBR = 97:1:2 Electrode B-1 Graphite B Series 1Graphite:CMC-Na:SBR = 97:1:2 Electrode C-1 Graphite C Series 1Graphite:CMC-Na:SBR = 97:1:2 Electrode D-1 Graphite D Series 1Graphite:CMC-Na:SBR = (Lot 2) 97:1:2 Electrode E-1 Graphite D —Graphite:CMC-Na:SBR = (Lot 2) 97:1.5:1.5 Electrode E-2 Graphite D —Graphite:CMC-Na:SBR = (Lot 1) 97:1.5:1.5 Electrode F-1 Graphite D —Graphite:CMC-Na:SBR = (Lot 1) 95:3.5:1.5 Electrode G-1 Graphite D —Graphite:CMC-Na:SBR = (Lot 1) 93:1.5:5.5 Electrode A-2 Graphite A Series2 Graphite:carbon fiber: CMC-Na:SBR = 96:1:1:2 Electrode A-2-2 GraphiteA Series 2 Graphite:carbon fiber: CMC-Na:SBR = 96:1:1:2 Electrode C-2Graphite C Series 2 Graphite:carbon fiber: CMC-Na:SBR = 96:1:1:2Electrode A-3 Graphite A — Graphite:carbon fiber:AB: CMC-Na:SBR = 96:1:Electrode A-4 Graphite A Series 3 Graphite:PVDF = 90:10 Electrode C-4Graphite C Series 3 Graphite:PVDF = 90:10 Electrode D-4 Graphite DSeries 3 Graphite:PVDF = 90:10 Electrode A-5 Graphite A Series 4Graphite:carbon fiber: PVDF = 89:1:10

Next, methods for fabricating the electrodes will be described. Thepolymerization degree of CMC-Na used for fabricating the electrodes was600 to 800, and the viscosity of a 1 wt % CMC-Na aqueous solution was inthe range from 300 Pa·s to 500 mPa·s.

Methods for fabricating Electrodes B-1, C-1, D-1, E-1, E-2, F-1, and G-1will be described. First, slurry was formed. Mixing was performed with aplanetary mixer. A container with a volume of 5 ml to 250 ml inclusive,was used for the mixing.

First, an aqueous solution was prepared in such a manner that CMC-Na wasuniformly dissolved in pure water. Then, the active material was weighedand the CMC-Na aqueous solution was added thereto.

Then, the mixture of these materials was kneaded in a mixer for 5minutes. The kneading was performed several times to form a paste. Here,kneading means mixing something so that it has a high viscosity.

Then, a 50 wt % SBR aqueous dispersion liquid was added to the mixture,and mixing was performed with a mixer for 5 minutes.

After that, pure water, which is a disperse medium, was added to themixture until it had a predetermined viscosity, and mixing was performedusing a mixer for 5 minutes once or twice. Through the above steps, theslurry was formed.

Subsequently, the slurry was applied to a current collector with the useof a blade. As the current collector, 18-μm-thick tolled copper foil wasused. The scanning speed of the blade was set to 10 mm/sec.

Subsequently, the coated electrode was dried using a hot plate at 50° C.in an air atmosphere for 30 minutes, and then, further drying wasperformed at 100° C. under a reduced pressure for 10 hours.

Though the above steps, Electrodes B-1, C-1, D-1, E-1, E-2, F-1, and G-1were fabricated.

Next, a fabricating method of Electrode A-1 will be described. First,slurry was formed. Mixing was performed with a planetary mixer. Acontainer with a volume of 1.4 L was used for the mixing.

First, pure water was added to CMC-Na to prepare an aqueous solution.Then, the active material was weighed and the CMC-Na aqueous solutionwas added thereto. The weight proportion of water in the total weight ofthe active material, CMC-Na, and water was set to approximately 28 wt %.

Then, the mixture of these materials was kneaded in a mixer forapproximately 40 minutes to form a paste. Here, kneading means mixingsomething so that it has a high viscosity.

Subsequently, an SBR aqueous dispersion liquid was added to the mixture,water was further added, and mixing was performed with a mixer for 20minutes.

Pure water serving as a dispersion medium was then added to the mixtureuntil a predetermined viscosity was obtained, and mixing was performedwith a mixer for 20 minutes.

Then, the pressure in the mixer containing this mixture was reduced anddegasification was performed for 20 minutes. The pressure was adjustedsuch that a pressure difference from the atmospheric pressure was 0.096Mpa or less. Through the above steps, the slurry was formed.

Subsequently, the slurry was applied to a current collector with the useof a continuous coater. An 18-μm-thick rolled copper foil was used asthe current collector. Two kinds of electrodes with different activematerial amounts were fabricated. Here, the electrode with an activematerial amount of approximately 10 mg/cm is Electrode A-1(a), and theelectrode with an active material amount of approximately 6 mg/cm² isElectrode A-1(b). The coating rates of Electrodes A-1(a) and A-1(b) were3 m/min. and 4 m/min., respectively.

Subsequently, the coated electrode was dried in a drying furnace.Electrode A-1(a) was dried at 80° C. in an air atmosphere for 48 secondsand then further dried at 100° C. in the air atmosphere for 32 seconds.Electrode A-1(b) was dried at 80° C. in an air atmosphere for 36 secondsand then further dried at 100° C. in the air atmosphere for 24 seconds.

After the drying in the drying furnace, further drying was performed at100° C. under a reduced pressure for 10 hours.

Through the above steps, Electrodes A-1(a) and A-1(b) were fabricated.

Next, methods for fabricating Electrodes A-2, C-2, and A-3 will bedescribed. First, shiny was formed. Mixing was performed with aplanetary mixer. A container with a volume of 5 ml to 250 ml inclusive,was used for the mixing.

First, graphite and carbon fiber were weighed, and water whose amount is36 wt % of the weight of graphite and the conductive additive was addedthereto. Then, Imputing was performed with a mixer to form paste-likeMixture 1 (Step 1). Here, kneading means mixing something so that it hasa high viscosity.

Then, CMC-Na was added to pure water to prepare a CMC-Na aqueoussolution (Step 2). After that, the prepared CMC-Na aqueous solution wasadded to Mixture 1 to form Mixture 2.

Subsequently, Mixture 2 was mixed in a mixer for 5 minutes several times(Step 3).

Then, SBR was added to pure water to prepare an SBR aqueous dispersionliquid (Step 4). After that, the SBR aqueous dispersion liquid was addedto mixed Mixture 2 to form Mixture 3. Formed Mixture 3 was mixed in amixer for 5 minutes (Step 5).

Then, pure water, which is a disperse medium, was added to Mixture 3until it had a predetermined viscosity, and mixing was performed using amixer for 5 minutes twice. Through the above steps, the slurry wasformed.

Subsequently, the slurry was applied to a current collector with the useof a blade. As the current collector, 18-μm-thick rolled copper foil wasused. The operating speed of the blade was set to 10 mm/sec.

Subsequently, the coated electrode was dried using a hot plate at 50° C.in an air atmosphere for 30 minutes, and then, further drying wasperformed at 100° C. under a reduced pressure for 10 hours.

Through the above steps, Electrodes A-2, C-2, and A-3 were fabricated.

A method for fabricating Electrode A-2-2 will be described below. Thecomposition condition for Electrode A-2-2 is the same as that forElectrode A-2 as in Table 2, and the fabricating method of ElectrodeA-2-2 is slightly different from that of Electrode A-2. First, slurrywas formed. Here, two kinds of electrodes with the same composition anddifferent active material amounts were fabricated. The electrode with anactive material amount of approximately 8 mg/cm² is Electrode A-2-2(a),and the electrode with an active material amount of approximately 9mg/cm² is Electrode A-2-2(b).

In both the cases of Electrodes A-2-2(a) and A-2-2(b), mixing wasperformed with a planetary mixer. A container with a volume of 1.4 L wasused for the mixing.

In both the cases of Electrodes A-2-2(a) and A-2-2(b), the activematerial was weighed and then carbon fiber powder and CMC-Na powder wereadded thereto.

Subsequently, water was added to the mixture and kneading was performedwith a mixer for approximately 40 minutes to form a paste. The amount ofwater added here was 25 wt % of the total weight of the mixture in thecase of Electrode A-2-2(a), and was 22 wt % in the case of ElectrodeA-2-2(b). Here, kneading means mixing something so that it has a highviscosity.

Subsequently, an SBR aqueous dispersion liquid was added to the mixture,water was further added, and mixing was performed with a mixer for 20minutes.

Pure water serving as a dispersion medium was then added to the mixtureuntil a predetermined viscosity was obtained, and mixing was performedwith a mixer for 20 minutes.

Then, the pressure in the mixer containing this mixture was reduced anddegasification was performed for 20 minutes. The pressure was adjustedsuch that a pressure difference from the atmospheric pressure was 0.096MPa or less. Through the above steps, the slurry was formed.

Subsequently, the slurry was applied to a current collector with the useof a continuous coater. An 18-μm-thick rolled copper foil was used asthe current collector. The coating rates of Electrodes A-2-2(a) andA-2-2(b) were 0.5 m/min. and 0.75 m/min., respectively.

Subsequently, the coated electrode was dried using a drying furnace. Thedrying was performed in an air atmosphere. Regarding the temperature andtime for the drying of Electrode A-2-2(a), the electrode was dried at50° C. for 180 seconds and then dried at 80° C. for 180 seconds.Regarding the temperature and time for foe drying of Electrode A-2-2(b),the electrode was dried at 50° C. for 120 seconds and then dried at 80°C. for 120 seconds.

After the drying in the drying furnace, further drying was performed at100° C. under a reduced pressure for 10 hours.

Through the above steps, Electrode A-2-2 was formed.

Then, Electrodes A-4, C-4, D-4, and A-5 were fabricated. In the cases ofElectrodes A-4, C-4, and D-4, graphite and PVDF were weighed such thateach composition was obtained as in Table 2 and mixed, and NMP was addedas a solvent to foe mixture to form slurry. In the case of ElectrodeA-5, graphite, carbon fiber, and PVDF were weighed such that thecomposition was obtained as in Table 2 and mixed, and NMP was added as asolvent to the mixture to form slurry.

Subsequently, a blade was used to apply the slurry to a currentcollector (18-μm-thick rolled copper foil), and then the solvent wasvolatilized by heat treatment. Consequently, Electrodes A-4, C-4, D-4,and A-5 were fabricated.

[ToF-SIMS Analysis]

Next, Electrodes A-1 and C-1 among the fabricated electrodes wereanalyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS).FIGS. 21A and 21B show mapping measurement results of Na ions, Na₂C₂HOions, and C₆H₅ ions. FIG. 21A shows the analysis results of ElectrodeA-1, and FIG. 21B shows the analysis results of Electrode C-1. Theobserved regions are each 19.5 μm square.

As shown in FIGS. 21A and 21B, Na and Na₂C₂HO that are presumably mainlyattributed to CMC-Na are observed in an entire surface of graphite. Inaddition, C₆H₅ that is presumably mainly attributed to SBR is observedin the entire surface of graphite. These results suggest that CMC-Na andSBR are dispersed well and distributed in the surface of graphite.

[Cross-Sectional TEM Observation]

Next, Electrode A-1 was sliced using a focused ion beam system (FIB) andthen a cross section thereof was observed with a transmission electronmicroscope (TEM). FIGS. 34A and 34B show TEM observation results. FIG.34A shows an observed part of a cross section of an electrode, and FIG.34B shows the part of the cross section of the electrode observed athigher magnification. As shown in FIG. 34B, graphite 151 is covered witha binder 152. The thickness of the binder 152 is estimated to beapproximately greater than or equal to 4 nm and less than or equal to 13nm. Note that a protective film 153 was formed over a surface of theelectrode for easier observation.

Next, vapor staining using osmium tetroxide was performed on ElectrodeG-1 to stain a double bond portion. After that, processing was performedusing FIB and a cross section of the electrode was observed with TEM.FIGS. 22A to 22C show TEM observation results. FIG. 22A shows a STEMimage. FIGS. 22B and 22C show Z contrast images. A film in contact witha surface of graphite was observed.

Next, FIG. 23A shows observation result obtained by high-angle annulardark field scanning transmission electron microscopy (HAADF-STEM), andFIGS. 23B to 23D show results obtained by performing element mappinganalysis by TEM energy dispersive X-ray (EDX) spectroscopy. It is foundthat the film in contact with the surface of graphite contained Os. Thisindicates that the film had a double bond before being stained. Thedouble bond is presumably attributed to SBR. Consequently, it issuggested that the film containing SBR was in contact with the surfaceof graphite.

Example 2

In this example, half cells were fabricated using the electrodes formedin Example 1, and the charge and discharge characteristics thereof weremeasured.

[Characteristics of Half Cells]

Each half cell was fabricated using the electrode formed in Example 1and a lithium metal as a counter electrode. The characteristics weremeasured with the use of a CR2032 coin-type storage battery (with adiameter of 20 mm and a height of 212 mm). For a separator, a stack ofpolypropylene and GF/C, which is Whatman glass-fiber filter paper, wasused. An electrolytic solution was formed in such a manner that lithiumhexafluorophosphate (LiPF₆) was dissolved at a concentration of 1 mol/Lin a solution in which ethylene carbonate (EC) and diethyl carbonate(DEC) were mixed at a volume ratio of 3:7. A positive electrode can anda negative electrode can were formed of stainless steel (SUS).

Next, the fabricated half cells were charged and discharged. Themeasurement temperature was 25° C. The conditions for charge anddischarge in the first and second cycles are as follows. The discharge(Li intercalation) was performed in the following manner constantcurrent discharge was performed at a rate of 0.1 C until the voltagedecreased and reached 0.01 V, and then, constant voltage discharge wasperformed at 0.01 V until the current value decreased and reached acurrent value corresponding to 0.01 C. As the charge (Lideintercalation), constant current charge was performed at a rate of 0.1C until the voltage increased and reached 1V.

Next, the constant current discharge rate in discharge (Liintercalation) was changed for each cycle in the third and later cycles,and the dependence on the rate was examined. Specifically, the dischargerate characteristics of 0.2 C, 0.3 C, 0.4 C, and 0.5 C were examined. Inaddition, in each cycle, constant current discharge was performed andthen constant voltage discharge was performed at 0.01 V under thecondition that the lower current limit was a current value of 0.01 C.Note that the conditions for charge (Li deintercalation) are the same asthose for the first and second cycles. The rate for charge (Lideintercalation) in the third and later cycles was 0.2 C.

Table 3 shows charge capacity with respect to discharge capacity in thefirst cycle as initial charge and discharge efficiency (chargecapacity÷discharge capacity×100[%]). Note that the active materialamounts and the densities shown in Table 3 indicate the actuallymeasured characteristics of the half cells. One electrode with twovalues of the active material amounts means that electrodes with thesame composition and different active material amounts were fabricated.

TABLE 3 Initial Active charge material and amount Electrode discharge[mg/ density efficiency cm²] [g/cm³] Series [%] Electrode A-1 9.7 1.3Series 1 97.4 6.1 1.2 Series 1 97.3 Electrode B-1 10.5 1.1 Series 1 96.7Electrode C-1 4.2 0.8 Series 1 94.8 Electrode D-1 5.8 0.9 Series 1 91.4Electrode E-1 4.9 0.8 — 92.8 Electrode E-2 7.3 0.9 — 91.6 Electrode F-17.1 0.9 — 92.9 Electrode G-1 7.9 0.9 — 92.7 Electrode A-2 4.6 1.3 Series2 96.9 5.7 1.1 Series 2 97.1 Electrode A-2-2 8.1 1.2 Series 2 96.9Electrode C-2 5.0 0.9 Series 2 94.2 Electrode A-3 6.1 0.8 — 96.1Electrode A-4 6.8 1.2 Series 3 95.6 Electrode C-4 8.0 0.9 Series 3 92.9Electrode D-4 7.5 1.0 Series 3 88.6 Electrode A-5 5.4 1.1 Series 4 90.5

As shown in Table 3, the smaller the specific surface area of graphiteis, the higher the initial charge and discharge efficiency is. In thecase where CMC-Na and SBR were used as binders, the initial charge anddischarge efficiency was higher than that in the case where PVDF wasused as a binder. FIG. 24A shows a graph where the specific surface areaof graphite and initial charge and discharge efficiency are plotted onthe horizontal axis and the vertical axis, respectively. Here, whendischarge capacity is C_(d) and charge capacity is C_(c) I(a) is definedby Mathematical Formula 10. FIG. 24B shows a graph where I(s) and thespecific surface area S [m²/g] are plotted on the horizontal axis andthe vertical axis, respectively. Here, discharge capacity refers tocapacity in Li intercalation, and charge capacity refers to capacity inLi deintercalation. Series 1 to 4 are the same as those in Table 2.

$\begin{matrix}{{I(s)} = {\frac{{Cd} - {Cc}}{Cd} \times 100}} & \left\lbrack {{Mathematical}\mspace{14mu}{Formula}\mspace{14mu} 10} \right\rbrack\end{matrix}$

FIG. 24B shows an approximate curve regarding I(s: Series 3) of Series3, that is, the condition where PVDF was used as a binder and carbonfiber was not added. When the specific surface area is S [m²/g], theapproximate curve can be expressed by Mathematical Formula 11.I(s:Series3)=1.4033×S+2.7318  [Mathematical Formula 11]

In contrast, I(s) of Series 1, that is, the condition where CMC-Na andSBR were used as binders was lower than that of Series 3. Here, byaligning I(g)=5.2 when Graphite C with a specific surface area of 2.73m²/g to I(s) in Mathematical Formula 11 when PVDF was used, S iscalculated to be 1.76. That is to say, when CMC-Na and SBR are used asbinders, for example, the effectual specific surface area in charge anddischarge corresponds to 1.76 m²/g. This means that, for example, aspecific surface area of 2.73−1.76=0.97 m²/g is presumably covered whenCMC-Na and SBR are used as binders.

Similarly, I(s) of Series 3 when Graphite D with a specific surface areaof 6.30 m²/g was used is 8.6. Here, by assigning 8.6 to I(s) inMathematical Formula 11 when PVDF was used, x is calculated to be 4.18.This means that, for example, a specific surface area of 6.30−4.18=2.12m²/g is presumably covered when CMC-Na and SBR are used as binders.

According to the above results, in the case where the weight of CMC-Naand the weight of SBR are respectively 1 wt % and 2 wt % of the sum ofthe weights of graphite, CMC-Na, and SBR, I(s) that corresponds to thestate where a surface corresponding to a specific surface area of 0.9m²/g to 2.2 m²/g is covered is estimated to be decreased.

Next, the electrodes using carbon fiber as conductive additives wereexamined. In the case of using PVDF as the binder and Graphite A with aspecific surface area of 1.49 m²/g, irreversible capacity was 9.5%. Incontrast, in the case of using CMC-Na and SBR as binders and Graphite Awith a specific surface area of 1.49 m²/g, irreversible capacity was3.0%, which is approximately equal to that when carbon fiber was notused. These results imply that when CMC-Na and SBR were used as binders,not only graphite but a surface of carbon fiber was possibly efficientlycovered, inhibiting the decomposition or the like of the electrolyticsolution at the surface of carbon fiber.

Next, the charge and discharge curves of the electrode using carbonfiber as the conductive additive and the electrode using carbon fiberand AB as the conductive additive are shown. FIGS. 25A1 to 23C2 show thecharge and discharge curves of the half cells using Electrodes A-1, A-2,and A-3. The discharge curves are the curve when the initial dischargerate was 0.1 C and the curve when the discharge rate was 0.3 C. Thecharge curve is the curve when the initial discharge rate was 0.1 C.

FIGS. 25A1 and 25A2 show the characteristics of the half cell usingElectrode A-2 with an active material amount of 5.1 mg/cm² and thecharacteristics of the half cell using Electrode A-2 with an activematerial amount of 6.3 mg/cm², respectively. FIGS. 25B1 and 25B2 showthe characteristics of the half cell using Electrode A-3 with an activematerial amount of 5.6 mg/cm² and the characteristics of the half cellusing Electrode A-3 with an active material amount of 6.5 mg/cm²,respectively. FIGS. 25C1 and 25C2 show the characteristics of the halfcell using Electrode A-1 with an active material amount of 6.0 mg/cm²and the characteristics of the half cell using Electrode A-1 with anactive material amount of 6.1 mg/cm², respectively.

It is found from the discharge (Li intercalation) curves in FIGS. 25A1to 25C2 that a voltage drop of the half cell using the electrodecontaining carbon fiber and the half cell using the electrode containingcarbon fiber and AB is smaller in the case of 0.3 C than in the case of0.1 C. Thus, the use of carbon fiber or carbon fiber and AB enabledfabrication of a favorable electrode with high electric conductivity andlow resistance.

Example 3

In this example, thin storage batteries were fabricated using theelectrodes formed in Example 1 for a negative electrode and the cyclecharacteristics thereof were measured.

Table 4 shows the conditions of the fabricated storage batteries.

TABLE 4 Capacity Negative ratio electrode Positive electrode Positiveelectrode reduction Electrolytic solution [%] Cell M Electrode A-2Positive electrode A Thermal reduction Electrolytic solution A 70.3 CellN Electrode C-2 Positive electrode A Thermal reduction Electrolyticsolution B 69.6 Cell O Electrode C-2 Positive electrode A Thermalreduction Electrolytic solution A 70.8 Cell P Electrode C-2 Positiveelectrode B Chemical reduction and Electrolytic solution A 71.7 thermalreduction Cell Q Electrode C-2 Positive electrode A Chemical reductionand Electrolytic solution A 66.0 thermal reduction Cell R Electrode A-1Positive electrode A Thermal reduction Electrolytic solution B 39.4 CellR′ Electrode A-1 Positive electrode A Thermal reduction Electrolyticsolution B 40.6 Cell S Electrode A-5 Positive electrode B Chemicalreduction Electrolytic solution B 62.2 Cell T Electrode A-2 Positiveelectrode C Not performed Electrolytic solution A 54.2[Fabrication of Positive Electrode]

As positive electrodes, Positive Electrodes A to D were used. Electrodeslurry for Positive Electrode A was formed using NMP as a solvent andcarbon-coated LiFePO₄ (hereinafter referred to as C/LiFePO₄), grapheneoxide, and PVDF (C/LiFePO₄:graphene oxide:PVDF=94.2:0.8:5.0 (wt %)). Thespecific surface area of C/LiFePO₄ used was approximately 25 m²/g.Electrode slurry for Positive Electrode B was formed using NMP as asolvent and LiFePO₄, graphene oxide, and PVDF (LiFePO₄:grapheneoxide:PVDF=94.4:0.6:5.0 (wt %)). The specific surface area of LiFePO₄used was approximately 9 m²/g. Electrode slurry for Positive Electrode Cwas formed using NMP as a solvent and LiCoO₂ with an average particlesize of 6.8 μm, AB, and PVDF (LiCoO₂:AB:PVDF=85.0:8.0:7.0 (wt %)).

The slurries for Positive Electrodes A and B were each applied to analuminum current collector (with a thickness of 20 μm) subjected toundercoating in advance and then were volatilized by heat treatment. Theslurry for Positive Electrode A was applied at 0.5 m/s, and the slurryfor Positive Electrode B was applied at 1 m/s. Heat treatment forPositive Electrode A was performed in such a manner that heating wasperformed at 80° C. for 2 minutes and then another heating was performedat 120° C. for 4 minutes. Heat treatment for Positive Electrode B wasperformed in such a manner that heating was performed at 80° C. underatmospheric pressure for 4 minutes. After that, reduction was performedby three conditions of “thermal reduction”, “chemical reduction andthermal reduction”, and “chemical reduction”.

“Thermal reduction” was performed at 170° C. under a reduced pressure(in vacuum) for 10 hours.

In the case of “chemical reduction”, graphene oxide was reduced byreaction in a solvent containing a reducing agent. The reductiontreatment was performed at 60° C. for 4.5 hours. Ascorbic acid was usedas the reducing agent. As the solvent, ethanol was used. Theconcentration of the reducing agent was 13.5 g/L. After that, washingwith ethanol was performed, and drying was performed at 70° C. for 10hours. The drying was performed in a vacuum atmosphere.

In the case of “chemical reduction and thermal reduction”, chemicalreduction was first performed, followed by thermal reduction. First,conditions for chemical reduction will be described. A solution used forthe reduction was prepared as follows: a solvent in which NMP and waterwere mixed at 9:1 was used, and ascorbic acid and LiOH were added to thesolvent to have a concentration of 77 mmol/L and 73 mmol/L,respectively. The reduction treatment was performed at 60° C. for 1hour. After that, washing with ethanol was performed, and drying wasperformed in an air atmosphere at room temperature. The drying wasperformed in a vacuum atmosphere. Next, conditions for thermal reductionwill be described. After the chemical reduction, the thermal reductionwas performed. The thermal reduction was performed at 170° C. under areduced pressure for 10 hours.

Subsequently, the positive electrode active material layer was pressedby a roll press method so as to be consolidated.

Positive Electrodes C and D were each also fabricated in such a mannerthat a formed paste was applied to an aluminum current collector (with athickness of 20 μm) subjected to undercoating in advance and drying wasperformed. Graphene oxide was not used for Positive Electrodes C and D;thus, reduction treatment was not performed.

Next, capacity values used for calculation of the ratio R of positiveelectrode capacity to negative electrode capacity will be described. Incalculating positive electrode capacity, 170 mAh/g and 137 mAh/g wereused as the capacity of LiFePO₄ and the capacity of LiCoO₂,respectively. In calculating negative electrode capacity, 372 mAh/g wasused as the capacity of graphite.

[Fabrication of Storage Batteries]

Next, single-layer thin storage batteries were fabricated using theformed positive and negative electrodes. An aluminum film covered with aheat sealing resin was used as an exterior body. In each storagebattery, the area of the positive electrode was 20.3 cm³ and the area ofthe negative electrode was 23.8 cm². As each separator, 25-μm-thickpolypropylene (PP) was used.

Electrolytic Solutions A and B were used as electrolytic solutions.Here, Electrolytic Solution A was formed in such a manner that anadditive such as VC was added to a solvent mainly containing EC, DEC,and ethyl methyl carbonate (EMC). In Electrolytic Solution A, lithiumhexafluorophosphate (LiPF₆) was dissolved at approximately 1.2 mol/L.

Electrolytic Solution B was formed in such a manner that VC as anadditive was added at 1 wt % to a solvent mainly containing EC and EMC.In Electrolytic Solution B, lithium hexafluorophosphate (LiPF₆) wasdissolved at approximately 1 mol/L.

Next, the fabricated storage batteries were subjected to aging. Notethat rates woe calculated using 170 mAh/g as a standard in the case ofusing LiFePO₄ as the positive electrode and 137 mAh/g as a standard inthe case of using LiCoO₂ as the positive electrode. Cells R, R′, and Swere charged at 0.01 C at 25° C. until the voltage increased and reached3.2 V, and then degasification and resealing were performed. After that,the cells were charged at 0.1 C at 25° C. until the voltage increasedand reached 4 V, and then discharged until the voltage decreased andreached 2 V. Cells M, N, O, P, and Q were charged at 0.01 C at 25° C.until the voltage increased and reached 3.2 V, and then degasificationand resealing were performed. Subsequently, the cells were charged at0.1 C at 25° C. until the voltage increased and reached 4 V, stored at40° C. for 24 hours, and then discharged at 25° C. until the voltagedecreased and reached 2 V. After that, the cells were charged anddischarged at 0.2 C twice. Cell T was charged at 0.01 C at 25° C. untilpower of 10 mAh/g was stored, and then degasification and resealing wereperformed. Subsequently, the cells were charged at 25° C. The charge wasperformed by CCCV, specifically, in such a manner that a voltage wasapplied at a constant current of 0.05 C until the voltage increased andreached 4.2 V and then a constant voltage of 4.2 V was maintained untilthe current value reached 0.01 C. After that, the cells were stored at40° C. for 24 hours, discharged at 25° C. until the voltage decreasedand reached 2 V, and charged and discharged at 0.2 C twice.

Next, the cycle characteristics of the fabricated thin storage batterieswere measured. Initial charge and discharge were performed at a constantcurrent of 0.2 C. Then, charge and discharge at a constant current of0.5 C were repeated for a cycle test. The upper voltage limit and thelower voltage limit of Cell T were 4.2 V and 2.5 V, respectively. Theupper voltage limit and the lower voltage limit of the other storagebatteries were 4.0 V and 2 V, respectively. The measurement temperaturewas 60° C. Charge and discharge were performed at 0.2 C in the 202thcycle and every 200 cycles after the 202th cycle.

FIGS. 26A and 26B and FIG. 27 are graphs where changes in capacity withincreasing number of cycles are plotted. Table 3 shows dischargecapacity in the second cycle, discharge capacity in the 300th cycle, andthe values each obtained by dividing discharge capacity in the 300thcycle by discharge capacity in the second cycle (C[300]/C[2]). Here, thecapacity of each storage battery per unit weight of the positiveelectrode active material was calculated. Each storage battery was putin a thermostatic bath where the temperature was controlled to be 60°C., and the cycle characteristics thereof were measured. As shown inFIGS. 26A and 26B and FIG. 27 , all the cells have favorable cyclecharacteristics. Furthermore, as shown in FIGS. 26A and 26B, all thecells are highly reliable storage batteries that can be subjected toapproximately 3000 charge and discharge cycles. Here, as shown in FIGS.26A and 26B, the capacities of Cells M, Q, R, N, and S decrease sharplyin portions surrounded by broken lines compared to other portions wherechanges are gradual. Periods when the capacities decrease sharplycorrespond to periods when interruption of power for the thermostaticbath occurs and the temperature decreases to approximately roomtemperature. Then, the power for the thermostatic bath was turned onagain, and accordingly, the capacities increased. After that, thecapacities gradually decreased again. In the long view, it can be saidthat the periods when the temperature decreases cause almost no adverseeffect

TABLE 5 C[2]: C[500]: Discharge Discharge capacity capacity in the inthe second 500th cycle cycle C[500]/ [mAh/g] [mAh/g] C[2] Cell M 147 1260.86 Cell N 136 115 0.85 Cell O 139 98 0.71 Cell P 140 99 0.71 Cell Q139 99 0.71 Cell R 142 132 0.93 Cell R′ 145 131 0.91 Cell S 125 99 0.79Cell T 133 128 0.96

The cell including the negative electrodes using CMC-Na and SBR asbinders has more favorable cycle characteristics than the cell includingthe negative electrode using PVDF. The smaller the specific surface areaof graphite is, the more excellent the cycle characteristics are.Furthermore, when the specific surface area of graphite is small, thedegradation of the characteristics with increasing number of charge anddischarge cycles can be inhibited even in the case of using PVDF as abinder.

First, the results of Cells N and S are considered. The ratios R ofpositive electrode capacity to negative electrode capacity aresubstantially equal to each other. The specific surface areas ofGraphite A used for Cell S and Graphite C used for Cell N were 1.49 m²/gand 2.73 m²/g, respectively. Here, assuming that in the case of usingCMC-Na and SBR as binders as considered in Example 2, a specific surfacearea of approximately 1 m²/g of Graphite C is covered, that is, thecharacteristics similar to those of graphite with a specific surfacearea of approximately 1.73 m²/g can be obtained, the characteristics ofCell N are presumably lower than those of Cell S unless the effects ofcarbon fiber and aging or the like are considered. The characteristicsof Cell N is actually better than those of Cell S, which suggests thatthe effect of covering carbon fiber with CMC-Na and SBR and the agingeffect are obtained, for example.

Next, the results of Cells R and N are considered. The ratio R of thepositive electrode capacity to the negative electrode capacity of Cell Ris lower than that of Cell N. Here, for example, when the ratio R ofpositive electrode capacity to negative electrode capacity is ½, thenegative electrode capacity is twice the positive electrode capacity.

Here, the doubled active material amount means a reduction in depth ofcharge and discharge of graphite by half and means doubled specificsurface area of graphite.

The reduction in depth of charge and discharge of graphite leads to adecrease in volume change due to expansion and contraction, so that, forexample, a coating film formed in initial charge is less likely to bedamaged and is not necessarily formed again. The loss of charge is thusprobably small and a reduction in capacity can be inhibited.Furthermore, the loss of a conductive path of the electrode due toexpansion and contraction can be presumably inhibited. This leads toinhibition of a decrease in capacity with increasing number of chargeand discharge cycles.

Here, the characteristics of Cell R are compared with those of Cell N.The ratio R of the positive electrode capacity to the negative electrodeopacity of Cell R is 0.57 times that of Cell N, and the amount ofgraphite in Cell R is 1.77 times (=1÷0.37) that in Cell N. The specificsurface area of graphite in Cell R is approximately 0.33 times that inCell R (note that carbon fiber is not in consideration). The equation1.77×0.33=0.97 is satisfied; therefore, a decrease in capacity of Cell Ris estimated to be approximately equal to that of Cell N if a reductionin capacity dominantly depends on only the specific surface area. Adecrease in capacity of Cell R, however, is actually significantlysmall.

These suggest that the depth of charge and discharge of graphite is alsoa parameter that considerably affects the cycle characteristics.

Next, reducing methods of the positive electrodes, the charge anddischarge characteristics, and the cycle characteristics will bedescribed. FIGS. 28A to 28C show the initial, second, 99th, and 999thcharge and discharge characteristics of Cells O, P, and Q. FIG. 29 showsa graph where the 999th charge and discharge characteristics of thecells are compared.

The gradient of a plateau region (flat potential region) in charge anddischarge is smaller in the cases of Cells P and Q (the cells subjectedto “chemical reduction and thermal reduction”) than in the case of CellO (the cell subjected to only “thermal reduction”). Thus, the positiveelectrodes of Cells P and Q presumably have lower resistance and aremore favorable than that of Cell O.

FIG. 27 shows the cycle characteristics of the storage battery usingLiCoO₂ for the positive electrode. Like in the case of using LiFePO₄ forthe positive electrode, favorable cycle characteristics were obtained byusing Graphite A with a small surface area. Furthermore, in the case ofusing LiCoO₂ for a positive electrode, the true density is high, whichincreases capacity per unit volume. That is, the occupied volume of thestorage battery can be small.

[Cross-Sectional TEM Observation and EELS Analysis]

Here, Cell R′ was subjected to 691 cycles of charge and discharge andthen disassembled, and its electrode was observed with TEM. Thedischarge capacity in the 691th cycle was 128.4 mAh/g. Note that thedisassembly of the cell, introduction to an FIB apparatus, andintroduction to the TEM were performed in an inert atmosphere.

FIGS. 30A to 30C and FIGS. 31A and 31B show observation results. RegionA and Region B each surrounded by a rectangle in FIG. 30A are enlargedfor further observation. FIGS. 30B and 30C are enlarged views of RegionA. FIGS. 31A and 31B are enlarged views of Region B. A protective film154 was formed for easier observation.

Next, regions whose cross sections were observed with TEM were analyzedby electron energy-loss spectroscopy (EELS). FIGS. 32A to 32C show EELSmeasurement results of the same portion as that of the observation imageshown in FIG. 30C. FIGS. 33A to 33C show EELS measurement results of thesame portion as that of the observation image shown in FIG. 31B.

FIG. 32A shows a portion subjected to linear analysis. FIG. 32B shows alinear analysts results of Li and C. FIG. 32C shows a linear analysisresult of O and F. The results imply that a film-like region that is incontact with a surface of graphite and includes Li, C, O, and F exists.

FIG. 33A shows a portion subjected to linear analysis. FIG. 33B shows alinear analysis results of Li and C. FIG. 33C shows a linear analysisresult of O and F. The results similarly imply that a film-like regionthat is in contact with a surface of graphite and includes Li, C, O, andF exists. The results also suggest that the concentration of Li is highat a surface of the film. When the distance from the surface of theobserved film-like region is r, the average value of the detectionintensity in EELS measurement in a region where r is greater than orequal to 0 nm and less than or equal to 30 nm is estimated to be atleast three times that in a region where r is greater than or equal to30 nm and less than or equal to 60 nm, as shown in FIG. 33B.

Example 4

[Low-Temperature Characteristics and Rate Characteristics]

Next, storage batteries were each fabricated using a negative electrodeformed under the condition of Electrode A-2-2 described in Example 1 andthe positive electrode described in Example 3, and the temperaturecharacteristics and rate characteristics thereof were measured. Thestorage battery using Positive Electrode A described in Example 3 isreferred to as Cell V, and the storage battery using Positive ElectrodeE including LiCoO₂ with an average particle size of 6.8 μm, AB, and PVDF(LiCoO₂:AB:PVDF=90.0:3.0:3.0 (wt %)) is referred to as Cell W. Table 6shows electrolytic solutions, the positive electrode reductionconditions, and the capacity ratios used for Cells V and W. For theconditions of the electrolytic solutions and the reduction conditions,the description in Example 3 can be referred to.

TABLE 6 Positive Capacity Negative Positive electrode Electrolytic ratioelectrode electrode reduction solution [%] Cell Electrode PositiveChemical Electrolytic 54.5 V A-2-2 electrode reduction solution A A andthermal reduction Cell Electrode Positive Not Electrolytic 76.1 W A-2-2electrode performed solution A E

As both of Cells V and W, thin storage batteries were fabricated usingthe positive electrode and the negative electrode in each of winch onesurface of a current collector is coated with an active material. Analuminum, film covered with a heat sealing resin was used as an exteriorbody. In each cell, the area of the positive electrode was 20.5 cm² andthe area of the negative electrode was 23.8 cm². As a separator,25-μm-thick polypropylene (PP) was used. In Cell V, 10 pairs of positiveand negative electrodes, in each of which one surface of the currentcollector is coated with the active material, were stacked. The positiveelectrode active material amount and the negative electrode activematerial amount in Cell V were 11.1 mg/cm² and 8.2 mg/cm², respectively.In Cell W, 6 pairs of positive and negative electrodes, in each of whichone surface of the current collector is coated with the active material,were stacked. The positive electrode active material amount and thenegative electrode active material amount in Cell W were 20.1 mg/cm² and9.4 mg/cm², respectively.

First, aging was performed. Cell V was subjected to aging under the sameconditions as those for Cell M in Example 3. Cell W was charged at 0.01C at 25° C. until power of 10 mAh/g was stored, and then degasificationand resealing were performed. Subsequently, the cell was charged at 25°C. The charge was performed by CCCV, specifically, in such a manner thata voltage was applied at a constant current of 0.05 C until the voltageincreased and reached 4.1 V and then a constant voltage of 4.1 V wasmaintained until the current value reached 0.01 C. After that, the cellswere stored at 40° C. for 24 hours, discharged at 25° C. until thevoltage decreased and reached 2 V, and charged and discharged at 0.2 Ctwice.

After the aging, the discharge rate characteristics and temperaturecharacteristics woe measured. First, the measurement conditions of thedischarge rate characteristics will be described. Charge was performedby CCCV, specifically, in such a manner that a voltage was applied at aconstant current of 0.05 C until the voltage increased and reached 4.1 Vand then a constant voltage of 4.1 V was maintained until the currentvalue reached 0.01 C. Discharge was performed at 0.2 C, 0.5 C, 1 C, and2 C. Note that the charge and discharge rates after the aging werecalculated based on the discharge capacity at the time of aging.

After the discharge rate characteristics were measured, the temperaturecharacteristics were measured using the same storage batteries. Themeasurement conditions of the temperature characteristics will bedescribed. Charge was performed at a constant current of 0.2 C at roomtemperature, and then discharge was performed at a constant current of0.2 C at 25° C., 0° C., −10° C., and −20° C. The upper charge voltagelimit of Cell V was 4 V, and that of Cell W was 4.1 V. The lower chargevoltage limit of Cell V was 2 V, and that of Cell W was 2.5 V.

FIGS. 35A and 35B show discharge rate characteristics, and FIGS. 36A and36B show temperature characteristics. Here, the capacity of each storagebattery per unit weight of the positive electrode active material wascalculated. FIG. 35A shows the rate characteristics of Cell V, and FIG.35B shows the rate characteristics of Cell W. FIG. 36A shows thetemperature characteristics of Cell V, and FIG. 36B shows thetemperature characteristics of Cell W. The use of the electrode formedaccording to one embodiment of the present invention as the negativeelectrode led to the favorable rate characteristics and temperaturecharacteristics of the storage batteries.

Example 5

In this example, storage batteries were each fabricated using theelectrode of one embodiment of the present invention, and thecharacteristics thereof were measured.

Cells 1 to 7 in Table 7 were fabricated as the storage batteries. Thepositive electrode active material amounts, the negative electrodeactive material amounts, and the capacity ratios R are as shown in Table7. Here, Cells 1 to 7 are thin storage batteries similar to those inFIG. 2 and FIGS. 3A and 3B in Embodiment 2.

[Fabrication of Negative Electrodes]

First, negative electrodes were fabricated. Graphite A shown in Table 1was used as each active material. In each electrode, the ratio ofgraphite:VGCF-H:GMC-Na:SBR was 96:1:1:2 (weight ratio).

TABLE 7 Positive Negative electrode electrode active active mateiralmateiral amount amount R [mg/cm²] [mg/cm²] [%] Cell 1 7.1 9.5 29 Cell 27.2 9.5 30 Cell 3 7.2 7.0 40 Cell 4 7.2 7.0 40 Cell 5 9.1 7.6 47 Cell 69.1 7.6 47 Cell 7 9.2 5.2 69 Cell 8 9.1 5.2 69

Slurry was formed using water as a solvent.

Mixing was performed with a planetary mixer. A container with a volumeof 1.4 L was used for the mixing. First, the active material was weighedand carbon fiber powder and CMC-Na powder were added thereto.

Subsequently, water was added to the mixture and kneading was performedwith a mixer for approximately 40 minutes to form paste-like Mixture 2.The amount of water added here was 24 wt % of the total weight of themixture. Here, kneading means mixing something 80 that it has a highviscosity.

Then, an SBR aqueous dispersion liquid was added to paste-like Mixture2, pure water was further added, and mixing was performed with a mixer.Water was further added and mixing was performed with the mixer, so thatMixture 3 was obtained.

Subsequently, obtained Mixture 3 was degassed under a reduced pressure.Through the above steps, the slurry was formed.

Subsequently, the slurry was applied to a current collector with the useof a continuous coater. An 18-nm-thick rolled copper foil was used asthe current collector. The active material amounts in the activematerial layers were adjusted to be the values shown in Table 7.

[Fabrication of Positive Electrodes]

Next, positive electrodes were fabricated using LiFePO₄ as activematerials.

Slurry was formed using NMP as a solvent and carbon-coated LiFePO₄(hereinafter referred to as C/LiFePO₄), graphene oxide, and PVDF(C/LiFePO₄:graphene oxide:PVDF=94.2:0.8:5.0 (wt %)). The specificsurface area of C/LiFePO₄ used was approximately 16 m²/g.

The shiny was applied to each current collector with a thickness of 20μm subjected to undercoating in advance. The active material amounts inthe active material layers were adjusted to be the values shown in Table7. Then, the solvents were volatilized by heat treatment. After that,“chemical reduction and thermal reduction” were performed, so that apositive electrode active material layer was formed over the currentcollector. The conditions described in Example 3 were used as theconditions for “chemical reduction and thermal reduction”.

Then, pressing was performed by a roll press method. Through the abovesteps, the positive electrodes shown in Table 7 were obtained.

[Fabrication of Storage Batteries]

Next, Cells 1 to 8 shown in Table 7 were fabricated using the negativeelectrodes and the positive electrodes that were formed. Single-layerthin storage batteries were fabricated. Here, “single-layer” means apair of positive and negative electrodes facing each other with aseparator therebetween.

An aluminum film covered with a heat sealing resin was used as anexterior body. In each cell, the area of a positive electrode was 20.5cm² and the area of the negative electrode was 23.8 cm². As eachseparator, 25-μm-thick polypropylene (PP) was used. As each electrolyticsolution, Electrolytic Solution A described in Example 3 was used.

Then, aging was performed. The conditions for the aging are as follows.Constant current charge was performed at 0.01 C at 25° C. until thevoltage increased and reached 3.2 V. Then, part of the exterior body wascut to open each cell, and degasification was performed. After that,resealing was performed. Next, constant current charge was performed at0.1 C at 25° C. until the voltage increased and reached 4 V. After that,each cell was stored at 40° C. for 24 hours. Subsequently, constantcurrent discharge was performed at 0.1 C at 25° C. until the voltagedecreased and reached 2 V. Then, constant current charge and dischargewere performed at 25° C. twice. Each constant current charge wasperformed until the voltage increased and reached 4 V, and each constantcurrent discharge was performed until the voltage decreased and reached2 V. Here, as to a rate C, 1 C corresponds to a current density per unitweight of the positive electrode active material of 170 mA/g.

[Measurement of Storage Batteries]

Next, the cycle characteristics of the fabricated thin storage batterieswere measured. Initial charge and discharge were performed at a constantcurrent of 0.2 C. Then, charge and discharge at a constant current of0.5 C were repeated for a cycle test. The upper voltage limit and thelower voltage limit were 4.0 V and 2 V, respectively. The measurementtemperature was 60° C. Charge and discharge were performed at 0.2 C inthe 202th cycle and every 200 cycles after the 202th cycle.

FIG. 37A is a graph where changes in capacity with increasing number ofcycles of Cells 1 to 4 are plotted, and FIG. 37B is a graph wherechanges in capacity with increasing number of cycles of Cells 5 to 8 areplotted. Table 8 shows the discharge capacities in the second cycle, the50th cycle, the 100th cycle, and the 300th cycle, which are referred toas C[2], C[50], C[100], and C[300], respectively. Table 9 shows thevalues obtained by dividing each of the discharge capacities in the 50thcycle, the 100th cycle, and the 300th cycle by the discharge capacity inthe second cycle (C[50]/C[2], C[100]/C[2], and C[300]/C[2]). Here, thecapacity of each storage battery per unit weight of the positiveelectrode active material was calculated.

TABLE 8 C[2]: C[50]: C[100]: C[300]: R [mAh/ [mAh/ [mAh/ [mAh/ [%] g] g]g] g] Cell 1 29 127 121 119 114 Cell 2 30 126 121 119 113 Cell 3 40 135131 129 124 Cell 4 40 134 131 129 124 Cell 5 47 132 128 127 123 Cell 647 134 130 128 123 Cell 7 69 136 132 131 128 Cell 8 69 138 134 132 127

TABLE 9 R C[50]/ C[100]/ C[300]/ [%] C[2] C[2] C[2] Cell 1 29 0.9560.937 0.896 Cell 2 30 0.956 0.938 0.897 Cell 3 40 0.967 0.953 0.924 Cell4 40 0.973 0.959 0.917 Cell 5 47 0.966 0.956 0.916 Cell 6 47 0.968 0.9530.928 Cell 7 69 0.974 0.963 0.929 Cell 8 69 0.973 0.960 0.933

First, the capacity in the second cycle is low in the case where thecapacity ratio is low. For example, the capacity of the cells whosecapacity ratio is 30% or less (Cells 1 and 2) was lower than or equal to127 mAh/g. The value C[100]/C[2] is small in the cells whose capacityratio is small, and is 0.94 or less in the cells whose capacity ratio is30% or less (Cells 1 and 2).

FIG. 38 is a graph showing the relation between the gradient of acapacity decrease and the capacity ratio of each cell. Here, a gradientα of a capacity decrease in the 11th to 30th cycles and a gradient β ofa capacity decrease in the 281th to 300th cycles are used as data. Here,α is expressed by the formula (the capacity in the 30th cycle—thecapacity in the 11th cycle)/20, and β is expressed by the formula (thecapacity in the 300th cycle—the capacity in the 281th cycle)/20.According to the graph, the gradient of a capacity decrease and thecapacity ratio strongly correlate to each other in earlier cycles, andbecome less likely to correlate to each other as the number of cyclesincreases. This result implies that in earlier cycles, as the capacityratio is lower (the surface area of the negative electrode activematerial with respect to the weight of the positive electrode activematerial is larger), the decomposition amount of the electrolyticsolution is larger. The result also suggests that as the number ofcycles increases, the cell with a high capacity ratio deteriorates byother factors such as a capacity decrease due to significant expansionand contraction of graphite as well as the decomposition of theelectrolytic solution.

Example 6

In this example, a bending test was performed on the thin storagebatteries described in Embodiment 2, and the charge and dischargecharacteristics thereof were measured.

Cells 9 and 10 shown in Table 9 were fabricated. The positive electrodeactive material amounts, the negative electrode names, the negativeelectrode active material amounts, and the capacity ratios R are asshown in Table 9. Here, Cells 9 and 10 are thin storage batteriessimilar to those in FIG. 2 and FIGS. 3A and 3B in Embodiment 2.

TABLE 10 Positive Negative electrode electrode active active materialNegative material amount electrode amount R [mg/cm²] name [mg/cm²] [%]Cell 9 8.1 Negative 11.3 56 electrode 1 Cell 10 7.0 Negative 11.0 64electrode 2[Fabrication of Negative Electrodes]

Negative Electrodes 1 and 2 shown in Table 9 were fabricated. Graphite Ashown in Table 1 was used for each negative electrode. Slurry forfabricating Negative Electrode 1 was formed using graphite, VGCF-H,CMC-Na, SBR, and water. In Negative Electrode 1, the ratio ofgraphite:VGCF-H:CMC-Na:SBR was $16:1:1:2 (weight ratio). Slurry forfabricating Negative Electrode 2 was formed using graphite, CMC-Na, SBR,and water. In Negative Electrode 2, the ratio of graphite:CMC-Na:SBR was97:1:2 (weight ratio).

Negative Electrodes 1 and 2 were fabricated using a fabricating methodsimilar to that of the negative electrode described in Example 5.

[Fabrication of Positive Electrodes]

Next, the positive electrodes were fabricated using LiFePO₄ as activematerials.

Slurry was formed using NMP as a solvent and carbon-coated LiFePO₄(hereinafter referred to as C/LiFePO₄), graphene oxide, and PVDF(C/LiFePO₄:graphene oxide:PVDF=94.2:0.8:5.0 (wt %)). The specificsurface area of C/LiFePO₄ used was approximately 27 m²/g.

The formed slurry was applied to each current collector with a thicknessof 20 μm subjected to undercoating in advance. The active materialamounts in the active material layers were adjusted to be the valuesshown in Table 7. Then, the solvents were volatilized by heat treatment.After that, “chemical reduction and thermal reduction” were performed,so that a positive electrode active material layer was formed over thecurrent collector. The conditions described in Example 3 were used asthe conditions for “chemical reduction and thermal reduction”.

Then, pressing was performed by a roll press method. Through the abovesteps, the positive electrodes shown in Table 9 were obtained.

[Fabrication of Storage Batteries]

Next, Cells 1 to 8 shown in Table 7 were fabricated as layered thinstorage batteries using the negative electrodes and the positiveelectrodes that were formed. Here, 10 pairs of positive and negativeelectrodes are provided. Each pair of electrodes face each other. Thepositive electrodes and the negative electrodes are alternately stackedand separated by a separator.

An aluminum film covered with a heat sealing resin was used as anexterior body. In each cell, the area of a positive electrode was 20.5cm² and the area of the negative electrode was 23.8 cm². As eachseparator, 25-μm-thick polypropylene (PP) was used. As each electrolyticsolution, Electrolytic Solution A described in Example 3 was used.

Next, aging was performed. Then, the cells were charged at 0.01 C at 25°C. until the voltage increased and reached 3.2 V, and thendegasification and resealing were performed. The cells were furthercharged at 0.1 C until the voltage increased and reached 4 V. Afterthat, the cells were stored at 40° C. for 24 hours. Subsequently, thecells were discharged at 0.2 C at 25° C. until the voltage decreased andreached 2 V. Then, charge and discharge were performed at 0.2 C twice.

[Measurement of Storage Batteries]

Next, a bending test was performed on each of the fabricated storagebatteries. Charge and discharge were performed before the bending test,after 1000 times of bending, after 3000 times of bending, after 6000times of bending, and after 10000 times of bending. FIG. 39 showsobtained discharge capacity. The horizontal axis in FIG. 39 representsthe number of times of bending.

The bending test was performed using a bend tester. FIG. 40 is aphotograph showing the appearance of the bend tester. The testerincludes a cylindrical supporting body with a radius of curvature of 40mm extending in the depth direction under a center portion where thestorage battery is placed. The tester also includes arms extending inthe right and left directions. End portions of the arms weremechanically connected to holding plates. By moving the end portions ofthe arms up or down, the holding plates can be bent along the supportingbody. The bending test of the storage battery was performed with thestorage battery sandwiched between the two holding plates. Thus, movingthe end portions of the arms up or down allows the storage battery to bebent along the cylindrical supporting body. Specifically, lowering theend portions of the arms permits the storage battery to be bent with aradius of curvature of 40 mm. Since the storage battery is bent whilebeing sandwiched between the two holding plates, unnecessary forceexcept bending force can be prevented from being applied to the storagebattery. Furthermore, bending force can be uniformly applied to thewhole storage battery.

The bending test was performed in the range of radius of curvature from40 mm to 150 mm at intervals of 10 seconds. The charge and dischargecharacteristics were measured at 25° C. after the storage battery wasdismounted from the tester. The charge and discharge were performed at0.2 C under the conditions that the upper voltage limit was 4.0 V andthe lower voltage limit was 2 V. The measurement temperature was 25° C.Note that discharge capacity (mAh/g) is a value per unit weight of thepositive electrode active material. Here, as to the rate C, 1 Ccorresponds to a current density per unit weight of the positiveelectrode active material of 170 mA/g.

FIG. 39 shows that the capacity of Cell 9 before the bending test(referred to as initial capacity) and the capacity thereof after 10000times of bending were 320 mAh and 311 mAh (97% of the initial capacity),respectively. In addition, the initial capacity of Cell 10 and thecapacity thereof after 10000 times of bending were 317 mAh and 305 mAh(96% of the initial capacity), respectively. A capacity decrease due to10000 times of bending was small in each cell. Thus, storage batteriesthat can be repeatedly bent were able to be fabricated using thenegative electrodes of one embodiment of the present invention. Thefabricated storage batteries were found to have favorable batterycharacteristics even after repeated bending.

This application is based on Japanese Patent Application serial no.2014-010689 filed with the Japan Patent Office on Jan. 23, 2014, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A power storage device comprising: a positiveelectrode; a negative electrode comprising: a negative active materialcomprising an active material particle; a carbon fiber; a first filmcovering the active material particle, the first film comprising a firstbinder and a second binder; and a second film covering the activematerial particle and the first film, the second film comprising Li, F,O and C; and an electrolyte, wherein a first portion of the second filmis in contact with the first film, wherein a second portion of thesecond film is in contact with the active material particle, wherein asurface of the carbon fiber is covered with the first binder and thesecond binder, and wherein the power storage device is capable of beingrepeatedly bent.
 2. The power storage device according to claim 1,wherein the second binder comprises a stylene monomer unit.
 3. The powerstorage device according to claim 1, wherein the second binder comprisesa butadiene monomer unit.
 4. The power storage device according to claim1, wherein a material of the active material particle is graphite. 5.The power storage device according to claim 1, wherein the first filmcomprises a region with a thickness of 2 nm or more and 20 nm or less.6. The power storage device according to claim 1, wherein the firstbinder is a cellulose derivative.
 7. A power storage device comprising:a positive electrode; a negative electrode comprising: a negative activematerial comprising an active material particle; a carbon fiber; a firstfilm covering the active material particle, the first film comprising afirst binder and a second binder; and a second film covering the activematerial particle and the first film, the second film comprising Li, F,O and C; and an electrolyte, wherein a first portion of the second filmis in contact with the first film, wherein a second portion of thesecond film is in contact with the active material particle, wherein asurface of the carbon fiber is covered with the first binder and thesecond binder, wherein the first binder is a water-soluble polymer,wherein a thickness of the first portion of the second film is thinnerthan a thickness of the second portion of the second film, and whereinthe power storage device is capable of being repeatedly bent.
 8. Thepower storage device according to claim 7, wherein the second bindercomprises a stylene monomer unit.
 9. The power storage device accordingto claim 7, wherein the second binder comprises a butadiene monomerunit.
 10. The power storage device according to claim 7, wherein amaterial of the active material particle is graphite.
 11. The powerstorage device according to claim 7, wherein the first film comprises aregion with a thickness of 2 nm or more and 20 nm or less.
 12. The powerstorage device according to claim 7, wherein the first binder is acellulose derivative.